You could add
agent of erebos. He's black, he's a zombie, and he clears out ENTIRE graveyards. All you would have to do is play
skeletal grimace while he's on the field and you've just eliminated a graveyard player's main deck engine.
Also here's another way to play: if you do use 2/2 black zombie tokens and then add corpse trader to the deck, you can sac your zombie tokens to force your opponent to discard SPECIFIC cards that YOU choose. This will completely eliminate your opponent's game plans, you'll get rid of their major threats early and still keep to your theme.
Here's an example, you just forced your opponent to discard
Geralf's masterpiece and you know its going to hit the board next turn. Trust me, this card will be a
nightmare for you to play against If you play
vile rebirth then you can get rid of that card, get a free creature, then sac the creature to your corpse trader. This will eliminate graveyard threats, produce a functional board state AND give you use out of your creatures. Or you can focus on the life drain aspect and add cards such as
mental agony. This will give you that black burn damage and it goes well with
liliana's caress.