Hey, I have a 50+3 Dark/Dark Corruptor so I can tell you it's nothing to write home about overall. Tarpatch is very satisfying, but otherwise I feel like I'm just spamming generic damage that happens to be black.
The -hit on enemies may be strong, but it doesn't feel satisfying. The Bonuses that Defender has over Corruptors/Controllers using their primaries as secondaries won't help (No shields, only one weakish heal) so the defender would be worse. I feel like I can also predict lackluster damage (by blaster standards) as dark/dark. I have a friend qith a Darkness Dominator which seems fine so I'd go with that or on of the melee AT's, sorry no hands on experience with fark melee.
Another option is recoloring Fire. I have a Fire/Kinetics with grey/black flames which can look really great. Grouping with a friend playing the same with a bright green looked amazing when we stood together with imps and auras.
