1. Barren Moor/Polluted Mire
The cycling lands are pretty awesome in the late game, especially in EDH. Once you've got all the mana you need, you can easily turn these into more useful cards. Both of these are pretty awesome inclusions in black decks.
2. Blood Seeker
Death by a thousand cuts is still death, right? This is a great addition to most decks. It slowly drains your opponents of life (or quickly if your opponent has tokens) and probably won't get removed for a while, and if it does, at least it's eating a removal spell instead of your general or another one of your important creatures.
3. Bog Initiate
Mono-black colour-fixing? What is this madness?! Seriously though, in a tri or five-colour deck, this card can allow you to run black-intensive spell without much worry of being short on black mana.
4. Choking Sands/Rancid Earth/Sinkhole
Ask anyone I play with and they'll tell you I love land destruction. Yes, it's dirty. Yes, it's unfair. However, despite being those two things, it is effective. Most decks don't have a way to recur their land cards from their yards so being able to blow and important one up at the right time can be crucial. Given that black can't really deal with noncreature permanents that well, these are great additions for dealing with problems.
6. Diabolic Edict/Geth's Verdict
People often discount the early game, but many problem creatures are played on the first few turns. From dealing with cheap commanders like Rhys the Redeemed and Gaddock Teeg to quick-drop creatures such as Serra Ascendant and Weathered Wayfarer, these cards are pretty efficient at their job. Finally, it also kills traditionally solo protected creatures like Rafiq of the Many and Uril, the Miststalker.
7. Eyeblight's Ending/Victim of Night
These are basically unconditional removal cards. The number of times that you won't be able to cast either of these can probably be counted on that guy on the right's fingers.
It's mono-black Journey to Nowhere. Yeah, it lets the creature keep all the counters and Auras attached to it, but mono-black exile is hard to come by, so I'll take what I can get.
9. Psychotic Episode
This works really well when played after an opponent tutors for a card since it forces them to waste another card to get it back. I also really like it because graveyards are very easily accessed in EDH, so discard spells aren't as valuable as other formats. Finally, if their commander is in your opponent's hand, they probably aren't going to be seeing it for a while.
10. Sadistic Glee
I just like this because it will eventually demand an answer, even if you don't do anything. Since EDH is a multiplayer format, your opponents can easily do all the heavy lifting for this card to work.
11. Shade's Form
Remember False Demise? This is the black version, only better. I don't even really care for the first bit (although it's pretty great for your own creatures) but the second part is great. Put it on an opponent's creature that you want or put it on your own that you don't want to die.
12. Tortured Existence
Finally, we come to a card that most people have seen, but I thought I'd include anyway since it is a common. Being able to constantly cycle through your creatures is pretty powerful, especially in reanimator decks. While not a powerhouse in its own right, it can easily be a very strong card when used correctly.