Common & Literal:
* as black as night
* as black as coal
* as black as ink
* as black as ebony
* as black as pitch
Figurative & Evocative:
* as black as a raven's wing
* as black as a panther's fur
* as black as a storm cloud
* as black as a bottomless pit
* as black as my heart (for dramatic effect)
Humorous:
* as black as a burnt marshmallow
* as black as a politician's promises
* as black as a student's coffee after finals week
The best completion depends on the context and what you want to convey.