We all know what black looks like, but technically it's defined by the absence of light. Total darkness. It can also mean an angry feeling or something wicked or hopeless. For example, “The traffic put me in a black mood.”
Black is most commonly used as an adjective, but you can also use it as a noun, like when you say, "The actors wore black." Synonyms when used to describe color include inky, raven, and sable. Synonyms when it means "hopeless" include dispiriting, lugubrious, and threatening.