Why Are My Cat's Balls Black?

If you mean "why is the fur on my cat's balls black?", then the answer would be genetics.
If the fur is not black, but the skin underneath is, this is merely pigmentation variation and quite common, especially among patterned cats or tabbies. If the balls look otherwise healthy, then I wouldn't worry about it, though your cat really should be neutered unless you plan to breed. It really is better for the cat.
If the balls are black because of some other cause, ie. Dried blood, matted feces, etc., then there is another problem altogether and the cat should be seen by a vet.