I would take soul mastery. power boost and soul drain are rather useful. Plus you are /dark and have the support/soft control thing covered. Power boost takes it even further. Cant type much as I have my youngest sitting in my lap trying to type as well atm lol. But yeah thats what id do.
Awesome thanks! That was all I really wanted to know. I appreciate the response! I'd talk more, but I have a sandwich in my laps. Yes. Plural.
Soul Mastery gives Power Build up and Soul Drain. I worry about using Soul Drain as a Corrupter, but Power Build up is a great power. Not my thing, but I see nothing wrong with it at all.
Mu Mastery gives Conserve Power, Power Sink, and Summon Adept (arguable the best Patron pet). I use this with my Fire/Kin Corrupter because Transference only misses at the absolute worst times, and I use so much endurance while blasting and buffing that it pays to have some extra recovery to stave off crashing.
Mace Mastery gives Scorpion Shield and Web envelope. The defense from Scorpion Shield will stack with your lovely -ToHit debuffs and make you harder to hit, and Web Envelope might be used with Darkest Night and Scorpion Shield to render enemies immobile and unable to hit you, or as another way of forcing enemies to stay in Tar Patch after you set off Rain of Fire and/or Fireball.
Too many alts to list.
I'm inclined to agree with Mr_Grumpums. Soul Drain didn't do much for my Ice Cor so I dropped it. I liked Power Build up. It added about 200 extra heal points. I had it on my Fire/Dark Cor. But, I wanted the immob to use with tar patch to keep the baddies from running. I usually play with a couple brutes who have aoes. Their powers aren't as effective if the baddies are running away. Plus it keeps them in place so I can run in and nuke'm. 
I'm Mu Mastery all the way. Fire/Dark is a complete end hog and Power Sink makes life *so* much easier. Plus it gives me the uber offensive combo of Inferno + Blue + Power Sink = Neverending Pain. The resistance shield is great for having layered defense types and it makes you nigh invulnerable to energy damage when stacked with DN and SF.
The version of Soul Drain that Soul Mastery gives you is the *worst* version in the entire game, so it's not really all that hot to take. But the shield is good and so is the newly added Power Boost, so it's second in my opinion.
A good Tar Patch is slotted with slows, so the AoE immob is relatively redundant IMO. The things that are resistant to slow are also usually resistant to immobs and when things are caught in a Tar Patch, they tend to die too fast to need an immob for them.
http://www.virtueverse.net/wiki/Massacre_Melanie -the original Fire/Dark Corruptor -
http://boards.cityofheroes.com/showthread.php?t=115217
The Guide to BURN
My search-fu is severely lacking, I must admit. I am trying to decide what would be a good APP for my fire/dark corr. I'm trying to take into account thematic fits (soul mastery would fit the character perfectly) vs practicality (mmmm aoe Immob).
So really, just how valuable would mako/mace's aoe immob be, factoring in tar patch & fearsome stare. Do i really need that extra control, or would it just be overkill?
Soul Mastery, as I said, would fit my character perfectly, what with being an evil demon blah blah blah blah, and I just cant see him whipping out a big ol mace to shoot webs out of. But on the other hand, there is focused accuracy and the like. I dont run into Oh Shi- moments enough to need anything other than Demonic. Leviathan mastery seems to be a decent middle ground, kind of, between theme and function.
But I could really use some practical advice.