Fire/Mental is a single target monster so that'd be my suggestion. I know a couple of people who have had great luck soloing things like pylons and AV's and seen many more post here, so if that's what you're planning to do I would go Fire. It also has ROF and Fireball, and there's Shockwave so you can have AOE if you want it.
I personally don't see the point in playing Dark Blast unless you're just a casual/solo/SO player who wants a blaster for concept and enjoys the controlly aspects, since you'll get better control and damage elsewhere, say by playing either a Fire Blaster or a Controller/Dominator (or a human Warshade.)
I think the Fortunata Widow is the closest thing to what I'm describing. Dominate, Subdue (Widow Subdue > Blaster Subdue) and Mental blast form a nice attack chain. less damage than a blaster and no self healing, but she'd be running double assault so the gap isn't too terrible, and she'd have WAAAYYY more defense potential.
My Fortunata is only 25, and I'm trying to narrow down my list of characters. Started way too many when I returned lol. It's either widow or dark/mind. Really prefer the style of dark/mind and the self heal, but I don't want to be useless and I know 6x leadership and AE control on the widow will always be worth something.
Fortunatas are indeed a bit more suited to what you're describing, with more control powers and massive defensive potential, but you certainly CAN build a ST-focused (or even ST-exclusive) blaster. If you go that route, you'll give up a good chunk of the damage mitigation for some more damage dealing. Building a blaster for ~+60% damage would be a neat trick, and on top of Defiance, you'd be floating near +100% damage before Aim/BU or any external buffs came into the picture. Tasty.
The real question is why you'd WANT to make your blaster ST-only, aside from PvP purposes, when blasters have the potential to do so much more AoE damage. If you're running "regular" missions, a Blaster's main contribution is the power to lay waste to entire groups of baddies simultaneously with large AoE attacks; the overall DPS of an attack chain where you spam AoEs vs. lots of guys will dwarf that of even the most optimized ST specialist. If you find yourself running lots of incarnate trials, or high-end TFs, or Mothership Raids, or similar areas where you have lots of higher level AV/EB level guys that will take a lot of time to whittle down, this is where having a dedicated ST specialist would shine. Soloing at something like +2/x1 would also make single-target damage a strong priority.
So yeah, you can do this, it's just a question of whether you want to do it. If it sounds fun, knock yourself out.
Rule number six of an empathy defender is NEVER underestimate a blaster's ability to die. I don't care if he has CM, Fort, both RAs, bubbles (both FF and Sonic), and is fighting next to a Storm defender with hurricane on. If there is a way to die in that situation, the blaster will find it.
I'd say, ignore the people who say 'this is the only way to succeed' and play what you want and build however you like. Don't let people who say 'You must only play optimized characters on optimized teams, vs optimized enemies' make your choices for you, they're just dreaming, since 90% of the game is not optimized. There's more to City of Life than AE missions and Taskforces.
Damage-boosted Dark should make a very potent soloing character. Any enemy you engage will be doomed to defeat and likely to never lay a glove on you. On a small team, single-target domination is every bit as fast as AoE survival. If you feel like it, you can use your second build to put in more AoE.
Go. Hunt. Kill Skuls. Try new things. Have FUN!
Be Well!
Fireheart
I have a level 28 Dark/MM Blaster, and it's been a lot of fun so far. With Tenebrous Tentacles and Psi Shockwave I was street sweeping groups in Steel Canyon from about level 12. The combination of immobilization, -to hit, and -recharge is good for incapacitating groups whether solo or teamed. Sure, you won't be the AoE king with this combo, but you'll still contribute a fair amount of AoE in a group and huge -regen from Drain Psyche.
I haven't even begun to make a final build for mine, so I'm not sure what direction I'll take, but from experience I like having ranged soft-cap on my blasters. It makes them very survivable, especially if you like hover-blasting or can lock down groups(as you can with TT), and for more damage you can always choose Musculature for your Alpha. As for Single Target blasting, use TT to lock down most of the group, if any still run at you, ST focus them til they're down, re-TT the group, and ST burn at will.
My advice is to do what you like, if you're having fun then stick with it, and worry about making a build later on. With IO set bonuses and Incarnate powers you'd have to try hard to make a bad character these days, or just be bad at using Mids. 
I don't know if they changed it, or I just misunderstood long long ago and never looked into it again, but I realized today that Dark Blast's Abyssal Gaze and Life drain are actually very worthwhile single target attacks! I had always thought of them as being like bean bag, just fluffy semi-controller type deals. Turns out they are a solid three attack combo. I also like the 'new' mental manipulation set, world of confusion looks fun!
Single target blaster that life drains rather quickly and runs a defensive aura.
(The info when building the character says World of Confusion doesn't even draw aggro!?)
So I started playing with mids, and I realized you can actually get every single damage bonus possible, on a dark/mind blaster. Five two percents, five two point five percents, five three percents, and all three of the four percent set bonuses. (+46.5% damage to all. O.O), oh... and I picked up assault...
Very interested in this character on paper, but as I browse the forums there's a lot of buzz about blasters soft capping defenses/resists. With Charged Armor I can make a decent showing on smash/lethal but that's about it, and her area damage is just about non-existent while everyone in atlas is doing AE/farms and huge mobs of enemies. Thus my question: Is it worth it to go for a single target kill specialist, or would it end up useless?
EDIT---
Actually I went with Fly and Teleport for flavor. I could drop one and get my hands on tough, as well. Not well slotted, but the two of them put me at better smash/lethal than just Charged Armor.