This sort of mutation is especially common in various sorts of cats, most notably leopards and jaguars, though lions and housecats are subject to it as well. It is, however, exceedinly rare in lions.
Affected animals are almost uniformly black in both skin and fur, though leopards and jaguars retain their characteristic markings, visible only up close due to the very low contrast. In these species the surrounding fur is black, but the markings are slightly darker, creating a shadowed appearance. Melanistic large cats are often referred to as black panthers. This mutation is due to a recessive gene in leopards, and to a dominant gene in jaguars and housecats. Consequently, the melanistic form of the jaguar is markedly more common than the melanistic leopard, though the latter is still observed with some regularity.
Unlike some color mutations, like albinism, melanism is not generally detrimental. It may in fact be slightly advantageous to nocturnal hunters like the jaguar. Recent research suggests that the mutation may also confer improved resistance to certain viruses. This is believed to be the reason that a very large percentage of south asian leopards are melanistic - a viral disease killed off many of the normally-colored ones, causing the melanism gene to be abnormally common in that population.
Melanistic housecats are popular as pets, despite (or perhaps because of) the superstitions associated with black cats. There is a slight difference between a melanistic cat and one which is merely black-furred, however - a black-furred cat will usually have a pink nose and pink foot pads, while in the melanistic morph these will be black or dark grey.