First, try not to trip over all those synergies lying around. Rad/Dark is a great combo!
On the Rad side, of course Radiation Infection and Enervating Field are your bread and butter (EF is an endurance hog, so slot appropriately). Even though you'll have tentacles to keep things standing still in the debuff fields, I like using Lingering Radiation as well for the recharge debuff; plus it's nice for bosses who don't get immobilized easily.
Tentacles is a great place for damage procs if you can find/afford them. I have 3 in mine and it makes a noticeable difference. Plus it's nice when a single target gets multi-procced, it just makes me giggle!
Choking Cloud is a "love it or hate it" power. I absolutely love it, but it really loses a lot of leverage if you're /dark blast. It can still be situationally useful, but since you're going to be positioning yourself for cones mostly, the PBAE nature of Choking Cloud is going to go to waste most of the time. I take it on all of my Radiation Emission characters, but when paired with Dark Blast I don't take it and don't miss it.
Hasten and Accelerate Metabolism play very nicely together. Very nicely.
Defensively, Radiation Emission slotted with tohit debuffs is a great survival tool (and pulling tool, too); offensively, the Tentacles/Nightfall combination has a tendency to melt groups of baddies with tons of damage spam.
Dark Blast can be a little light on single-target damage, and doesn't get Aim, so you may want to slot up Dark Blast and possibly Gloom or Moonbeam; though if you have the veteran attack powers you may find you get by without Gloom or Nightfall (Radiation Infection and Enervating Field make the veteran powers much more reliable and effective).
Enjoy your new 'fender!
Thank you for your very informative response! What are your thoughts on Dark Pit, on paper it least it seems very good. Is it worth slotting up? I believe it has a -acc, not sure if that is correct or not. Thank you again for your help!
I've dabbled with Dark Pit but, on its own, it didn't really do much for me. There's no accuracy debuff component to it, just a straight minion-only stun which lasts between about 9 and 12 seconds base, depending on your level. And it's got a one-minute recharge, and a lower base accuracy—though in this case Radiation Infection would help with that last bit.
Where Dark Pit really has potential is if you have another stun to stack it with, like Thunderclap (Storm Summoning) or Oppressive Gloom (epic power pool).
Tentacles and Nightfall both do a base 9.38 tohit debuff, and that's on top of Radiation Infection's 31.25 (base) to 49.87 (slotted) tohit debuff. My theory is that if things are unable to move toward you, their accuracy is floored, and they're suffering from a -75% recharge debuff (remember I said I love Lingering Radiation?) ... well, a short-duration, minion-only stun isn't bringing a whole lot to the mix there. 
My absolute best advice is to copy yourself over to the test server once you have some levels and try different options out for yourself. Not everyone plays the same, and you may decide you like <insert power name here> after all.
Defender Radiation Infection will wipe out targets' to-hit, while Tenebrous Tentacles should keep the targets from escaping. So once you've got those two set up, you might concentrate on damage boosting powers: AM, EF, and blasts of course.
Life Drain is of questionable use since Radiant Aura already gives you a top notch self heal.
With the cone heaviness of /Dark I find that trying to stay in PBAoE range a little bit clumsy. I'd skip Choking Cloud for that reason, but I wouldn't call it a mistake to take it anyway.
One possible team build:
(This is not a leveling build--I'd certainly pick up Gloom on the way to 20 and respec later.)
Hero Plan by Mids' Hero Designer 1.401
http://www.cohplanner.com/
[u]Click this DataLink to open the build![u]
Hero Profile:
Level 1: Radiation Infection -- DarkWD-ToHitDeb:50(A), DarkWD-ToHitDeb/Rchg:50(3), DarkWD-ToHitdeb/Rchg/EndRdx:50(3), DarkWD-ToHitDeb/EndRdx:50(5), LdyGrey-%Dam:50(5), Achilles-ResDeb%:20(11)
Level 1: Dark Blast -- Decim-Acc/Dmg:40(A), Decim-Dmg/EndRdx:40(7), Decim-Dmg/Rchg:40(7), Decim-Acc/EndRdx/Rchg:40(9), Decim-Acc/Dmg/Rchg:40(11), Dev'n-Hold%:50(13)
Level 2: Radiant Aura -- Dct'dW-Heal/EndRdx:40(A), Dct'dW-EndRdx/Rchg:40(29), Dct'dW-Heal/Rchg:40(40), Dct'dW-Heal/EndRdx/Rchg:40(42), Dct'dW-Heal:40(42)
Level 4: Accelerate Metabolism -- RechRdx-I:40(A), P'Shift-EndMod/Rchg:40(9), P'Shift-End%:40(17), Efficacy-EndMod/Rchg:40(17), Efficacy-EndMod/Acc/Rchg:40(19), Efficacy-EndMod/EndRdx:40(19)
Level 6: Combat Jumping -- ULeap-Stlth:50(A), Krma-ResKB:30(29)
Level 8: Hurdle -- Jump-I:40(A)
Level 10: Dark Pit -- Stpfy-Acc/Rchg:40(A), Stpfy-EndRdx/Stun:50(13), Stpfy-Acc/EndRdx:50(15), Stpfy-Stun/Rng:50(15), Stpfy-Acc/Stun/Rchg:50(21), Rope-Acc/Rchg:40(27)
Level 12: Swift -- Run-I:40(A)
Level 14: Super Jump -- Jump-I:40(A)
Level 16: Tenebrous Tentacles -- SipInsght-Acc/EndRdx/Rchg:40(A), SipInsght-Acc/Rchg:40(23), Det'tn-Dmg/EndRdx:40(23), Posi-Dmg/Rchg:40(25), Det'tn-Acc/Dmg:40(25), Posi-Dmg/EndRdx:40(27)
Level 18: Lingering Radiation -- Acc-I:30(A), RechRdx-I:30(21), RechRdx-I:40(46)
Level 20: Stamina -- EndMod-I:40(A), P'Shift-EndMod:40(33), P'Shift-End%:40(46)
Level 22: Night Fall -- Posi-Acc/Dmg:30(A), Posi-Dmg/EndRdx:30(33), Posi-Dmg/Rchg:30(33), Posi-Dmg/Rng:30(34), Posi-Acc/Dmg/EndRdx:30(34), SipInsght-Acc/Rchg:30(34)
Level 24: Enervating Field -- EndRdx-I:40(A), EndRdx-I:40(36)
Level 26: Hasten -- RechRdx-I:45(A), RechRdx-I:45(36), RechRdx-I:45(36)
Level 28: Assault -- EndRdx-I:40(A)
Level 30: Tactics -- AdjTgt-ToHit:30(A), AdjTgt-ToHit/Rchg:30(31), AdjTgt-ToHit/EndRdx/Rchg:30(31), AdjTgt-EndRdx/Rchg:30(31), AdjTgt-ToHit/EndRdx:30(40)
Level 32: EM Pulse -- BasGaze-Acc/Hold:30(A), BasGaze-Acc/Rchg:30(37), BasGaze-Rchg/Hold:30(37), BasGaze-EndRdx/Rchg/Hold:30(37)
Level 35: Mutation -- EndRdx-I:40(A)
Level 38: Vengeance -- RedFtn-Def/EndRdx:40(A), RedFtn-Def/Rchg:40(39), RedFtn-EndRdx/Rchg:40(39), RedFtn-Def/EndRdx/Rchg:40(39), RedFtn-Def:40(40)
Level 41: Fallout -- Det'tn-Dmg/Rchg:40(A), Dmg-I:40(42), Dmg-I:40(43), RechRdx-I:40(43), RechRdx-I:40(43)
Level 44: Oppressive Gloom -- Stpfy-Acc/Rchg:40(A), Stpfy-EndRdx/Stun:40(45), Stpfy-Acc/EndRdx:40(45), Stpfy-Stun/Rng:40(45), Stpfy-Acc/Stun/Rchg:40(46)
Level 47: Dark Embrace -- S'fstPrt-ResDam/Def+:30(A), TtmC'tng-ResDam/EndRdx:30(48), S'fstPrt-ResDam/EndRdx:30(48), RctvArm-ResDam/EndRdx:40(48)
Level 49: Soul Drain -- C'ngBlow-Acc/Rchg:40(A), Erad-Acc/Rchg:30(50), Erad-Acc/Dmg/Rchg:30(50), RechRdx-I:40(50)
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As stated above: Choking Cloud doesn't play well with the coney disposition of Rad/Dark. (This build has Oppressive Gloom, which you might say the same thing about, but that has the advantage of stacking with Dark Pit.)
The goal of this build is to get a lot of recharge, at not-quite-flabbergast-inducing expenditures, though some of the IOs can be rather pricey if you're not used to plunking down 5 million for a recipe.
A few explanations:
RI procs are fun and useful, particularly the -res stacked with EF and Assault.
Dark Blast A slightly cheaper (and less damaging) variant would have Entropic Chaos instead of 5 Decimations. The hold proc is quite handy.
AM The +end proc could be subbed out for a Performance Shifter EndMod (or EndMod/xx) to cap out the end recovery. The +end proc is not terribly effective here, but gives you the bonus 10% of the time of getting AM "for free" and sometimes this really helps when you've cast EMP. (likewise for Stamina proc)
CJ Different travel powers can certainly be selected, including Superspeed to gain another power choice (and potential invisibility). The -kb universal travel IO is quite a bit more expensive than the Karma, though. I highly recommend some sort of "free" stealth for setting up RI best when you need to.
Dark Pit When you get into your late 20s and 30s, you'll be glad to be able to stun minions, like Quicksand casters and stunners.
TT Frankenslotted for ED-capped stats. You won't need extra immobilize because you'll be casting this so often.
Hasten almost necessary to build enjoyment and constant flow of cones.
Assault and Tactics Tactics is a bit redundant given RI--but it has a few other benefits (perception) and works during that long RI cast. Assault is amplified by EF. Both are a means to get to Vengeance.
To be continued...
... Fallout, Vengeance, Mutation These, like the Leadership powers above, are not necessary. But ... with the reduction of death penalties, I find myself mildly pleased when a teammate dies. It's just so fruitful. Vengeance is actually a pretty good set mule (though heal uniques would be wasted in it); you can tailor it to your IO needs. With ED-capped defense enhancement it brings your (surviving) teammates 6% from the defense soft-cap.
None of the Epic defender pools match perfectly with Rad/Dark. Psychic Mastery is probably the best--offering a hold (Great set bonuses for 4 Basilisk's or 6 Lockdowns. Not horrible set bonuses for 4 Ghost Widow's.), a tricky-to-use positioning tool, and group sleep (great for setting up your anchors/LR if those powers aren't proc'd out).
Dark has the draw of a inconvenient but stackable stun and Soul Drain.
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Hasten/AM are not quite perma- in this build, but that likely won't really make much difference.
IO Sets
You can build with IOs in mind for something other than recharge, namely Range and AoE defense bonuses. This is slot intensive (almost all decent def bonuses come from 5th and 6th slots), expensive (for the Breath of the Zephyr -kb recipe, the one, non-slot intensive Range + AoE defense option), and would come at the cost of recharge bonuses. Defense also is somewhat redundant with RI, but can be counted on with spread spawns, adds, dead-anchor-syndrome, and purple-patch issues, which are all trouble for RI (though Dark Pit, EMP, LR, and tentacles can all help this).
In a defense option Maneuvers would be swapped out for Assault, and you'd pick up sets like Thunderstrike, Red Fortune (to the 6th slot), Lethargic Repose [in Mass Hypnosis], Detonation, BotZ, and even overslot things like Stamina (with Performance Shifter) and even attacks with the accurate to hit debuff sets.
Other Powers
I'd regard the Leadership (work done without any effort!) and the corpse powers (fun!) as optional.
A build that plays on small teams or solos should pick up more single target material (Devastation Hold proc in more than one can be quite nice), maybe even using Life Drain as a ST attack rather than a heal.
I like Torrent as a (bludgeon- not scalpel-like) positioning tool, and it can take a good set in Kinetic Crash, which is cheap in terms of influence, though expensive in terms of slots.
Hello there Defenders. With the upcoming Double XP weekend (yes it is still a ways away) I was thinking of finally leveling a Defender (I've mainly been a Scrapper/Blaster player). I really want to level a Radiation/Dark defender, for some reason it really appeals to me. Anyone have any build suggestions or guides? Any advice on a Rad/Dark would be much appreciated!