Why are we obsessing over raw numbers and ignoring all of the secondary effects? Dark Blast is never going to be on par with, say, Fire Blast because Fire's secondary effect is more damage. That is all it does. Meanwhile, Dark has other tricks, debuffing ToHit. Look, I played this game for years way back, and in my opinion power sets were notappealing for their raw damage, but for the secondary effects and how you could leverage them in your favor. I played Elec Blast, and I knew I was never going to hit Fire levels of damage, but I leaned into the -End and ended up powerful in a different way. You can't just ignore all of the tricks that each set has.
And, on that note, I play a Dark/Poison Corruptor right now. It works just fine in my opinion, that cone immobilize is the exact reason why I wanted to play the set in the first place. It's not there for damage, there are other powers in the set for that purpose. Seriously, back in the old days people understood how to exploit all the tools in each powerset and find out what makes each set powerful. If you want raw damage, play fire. If you want to do something interesting then play another set, experiment, change your approach, and THINK.