I think black has enough removal, but just in case, here is one more
Infected wound
Instant
Destroy target creature that took damage this turn

We could also give black some more discard. Right now, we only have Black Cat
Idea steal
sorcery
Target player discards a card, and you draw a card

How about a small board wipe? (probably uncommon)
Dust the floor
Sorcery
All creatures get -1/-1 until end of turn. Whenever a creature dies this turn, you gain 1 life.

Black occasionally gets direct life loss
Chronic pains
Sorcery
Target player loses 4 life. Shuffle ~ into its owner's deck.

The direct life loss could also take a form similar to Stab wound, although we already have Gremlin's Hunt doing something similar
Contaminated bond (reprint)
Enchantment - aura
Enchant creature
Whenever enchanted creature attacks or blocks, its controller loses 3 life.

If we do not want to do another gremlin hunt, how about a positive enchantments for black?
Scare pox
Enchantment - aura
Enchant creature
Enchanted creature has menace.
When enchanted creature dies, you may put it on top of its owner's library.