Color:
* Dark
* Jet
* Ebony
* Sable
* Pitch
* Coal
* Midnight
* Onyx
* Sooty
* Charcoal
* Dark blue (for very dark blues)
Figurative/Symbolic:
* Gloomy
* Mournful
* Sinister
* Evil
* Dark (as in "dark secrets")
* Shadowy
Other:
* Negro (dated, avoid using this term)
* African-American (cultural term, more appropriate than "black" in some contexts)
The best word to use depends on the specific situation and the desired tone.