Black is one of the three basic colors of cats which I would call "pure." The other two are red and white. All other colors and color patterns of cats are a combination of any two or three of these basic colors.
The differences are all a matter of genetics, a very complicated study of the genes which are carried down from cat-to-cat. Cats with a dominant black gene are often (but not always) pure black in appearance. A cat with two parents, both possessed of a dominant black gene, will almost always be pure black. However, if either of these cats has a recessive red gene, the child may also carry it, a fact which accounts for the fact that sometimes black cats appear "rusty" in the sun.