Real World Comparisons
Raven or crow's wing
Animal fur (notably: panther, bear, cat, horse)
The proverbial sheep
Black jelly beans
Cast iron pans
Kiwi fruit seeds
Eight ball (in pool)
Bikers leather (jacket, pants, etc)
Synonyms for Black: jet, ebon, ebony, inky, obsidian, onyx, sable, sooty
Make every detail count. Colors are powerful descriptors, not fillers. Make sure that if you use a comparison or contrast to highlight a color, you choose the right one. Look at the setting and atmosphere you are working to create, then draw from the viewpoint character or narrator's history, education and past experiences to find the right fit.
A poor example:
Once Wendy had loaded up at the buffet she sashayed to our table, smiling like she actually thought she belonged. She'd chosen a short ebony dress as her outfit this evening, which made her bloated white legs as appealing as soiled hospital linen.
What's wrong with this example?
The emphasis is more on how the black skirt makes her legs look, rather than describe the skirt itself. If the black object is important, don't let other things overrun the description.
A strong example:
Once Wendy had loaded up at the buffet she sashayed to our table, smiling like she actually thought she belonged. As she stood there balancing a heaped plate, I took a closer look at the ridiculous ebony dress encasing her chubby form. What was she thinking--a shiny number like that should make the wearer look sleek, not a dung beetle clinging to its precious meal.
Why is this example better?
It provides a simile that gives an apt image and also really says something about the nastiness of the person making the comparison.