Dark Blast has garnered a reputation as being one of the blasting sets with the least damage output, which is why many folks don't take it for Corruptors.
Whether there's any truth in that rumour, I don't know. I've not used the powerset myself.
I use it on my /thermal. The idea being to debuff with the primary and buff/heal/debuff with the secondary. I'm sure a different secondary could push the damage up a bit.
My MA IDs:
Fiend Space: 211464
It's not that bad damage wise. Especially when you can take good position for the cones to hit as many as possible for some decent multiple DoT damage.
It is a smooth set to play with good utility and great synergy with /rad, /dark and /cold for some exceptional (de)buffing. Just don't expect to be a damage monster. If you want that play fire/ or sonic/
But all in all nothing wrong really with dark blast. 
As others have said, the main reason is just that it's quite low damage. A lot of people want to solo quickly, act as a team damage dealer, or be able to actually hurt people in PvP, and there are better sets for those things.
Where the set can shine, however, is on a more support-based character. The obviously synergy is with Dark Miasma, because you can stack a lot of tohit debuffs between the two sets. Aside from the debuffing, it also has some control elements.
So I wouldn't say there's anything wrong with the set, it's just that it does something different from the more popular sets. On the hero side, Dark Blast is actually quite popular amongst Defenders, probably because of its support characteristics, and the fact that Defenders get better debuffs from it, and won't be doing fantastic damage regardless.
It's DPS is ok, has pitiful burst damage imo. Would benefit from a few tweaks, something like the Soul Mastery AoE instead of Dark Pit and Aim instead of something like the Ranged Heal.
One of the favourite dueling builds in the US is dark/cold the idea being you have an uninteruptable heal, but generally the set is weak for pvp apart from /dark and /rad for flooring tohit.
Edit: Automatically assume pvp >.< from a PvE perspective the set has nice AoEs which give a nice debuffage I love it on my dark/dark defender and as said previously very nice in conjunction with other -acc sets.
thePhilosopher Martial Arts/Regen/Fire Scrapper
theRegulator Empathy/Energy/Soul Defender
Total Inertia Ice Blast/Kinetics/Psy Corruptor
Total Inferno Ice Blast/Thermal/Leviathan Corruptor
I have a level 50 Dark/Dark Corrupter which is my favourite and first toon 
In PvE the cone Imob from Tentacles and dmge from Night Fall keep alot of mobs nicely grouped, paired with Dark miasma you can kill entire mobs without taking any damage at all, but if you do, twilight grasp should easily be able to heal you back to full HP.
In PvP i find dark blast pretty good against certain appoinats and builds, it does lack burst damage but the DoT from the attacks makes things like aid self un-usable. I can defeat most /elec brutes with ease, and paired with Dark miasma is a pretty good powerset imo. Also works well in Team PvP when paired with dark miasma, fear, darkest night, tar patch, a nice hold, etc
But i would like to see a Aim included in the powerset, and probaly switched with life drain because imo its not a very good power and the heal isnt that great.
Shadow Crown
IS there something wrong with this powerset?
I am using it now, and was a bit surprised to see the sniper shot a 2nd level pwoer. I have never seen another dark blast Corrupter, is tehre something wrong with it or is it bad in some way?