Dark Miasma doesn't play terribly well with IOs. Many times, to get the set bonuses you want from sets, you end up needing more slots than you would otherwise need. As such, I usually end up frankenslotting to maximize the effects I want or to cut a slot here and there.
Dampened Spirits is your friend here since it has a lot of the effects you want.
I usually put Glimpse of the Abyss (minus the proc) into Fearsome Stare and fill the last slot with a ToHit Debuff enhancement (in the case of my Defender, a Lysosome HO)
Rad Blast is easier to do:
Obliteration in Irradiate (5 IOs from the set + 1 generic recharge IO)
Positron's Blast in Electron Haze (which I suggest you get) and Neutron Bomb (skip the proc, substitute in a generic recharge IO)
Sting of the Manticore in Proton Volley
Frankenslot Cosmic Burst for Damage, Accuracy and Recharge if you can't afford to put Apocalypse (minus the proc) in it.
The only outliers are Neutrino Bolt and X-Ray Beam.
In Neutrino Bolt, it's suggested that you frankenslot in two Dam/Acc IOs, put in the Achilles Heel proc, and then follow up with any 3 damage procs (any combination of Touch of Lady Grey, Shield Breaker, Apocalypse and Gladiator's Javelin). The Apocalypse proc is highly recommended since it both deals more damage and fires more often than other available procs.
X-ray beam doesn't need quite the same treatment, but it's still very worthwhile to load it up with procs after some normal slotting or frankenslotting.
I do highly suggest that you get Electron Haze and put as many slots into it as you can. It's the best AoE in an AoE-heavy set. It does more damage, recharges quicker and uses less endurance than Irradiate. It's really good.
I also find that, on a Dark/Rad, it's fairly painless to drop Petrifying Gaze. Rad Blast has Cosmic Burst, which fills in nicely for when you want single-target control.
I created a dark/rad about 2 weeks ago, and he is almost lvl 50, lvl 46 now, and I was seeing if anyone could help me out with a build for him, like for IOs and slotting and all that stuff. He has all but the Black Hole for the dark powers, and all but the cone attack, and the nova attack from the blast set. Could anyone help me make a good build with no Purples please, or maybe just purple procs at the most, no full on purples please, not going to be a full on main toon, got 2 toons purpled out on both their builds and takes to damn long to purple out another.