Hey oldtimer, you pegged it. Melanism is the opposite of albinism and occurs with about the same frequency. The genetic basis is not clearly understood, but inbreeding is considered partially responsible.
Melanin is the dark pigmentation which is responsible for the tanning process which occurs in humans when they're exposed to sunlight. In people it is actually a reaction to damage of the skin cells. A melanistic animal, however, has an increased amount of this black or nearly black pigmentation in the skin, feathers, hair, or other outer tissues. This is caused by a recessive gene just as in albinism. Many reptile breeders develop unique coloration patterns by taking advantage of this genetic anomaly.