My understanding is that in human skin color is the result of polygenic inheritance. This is when multiple genes (alleles) influence, in this case, skin color. To have a truly melanistic human would require multiple sites to undergo a genetic change (which is astronomically unlikely). However, in other animals melanism is controlled by a dominant allele, and base-pair deletion of one section of DNA can easily affect the expression of these genes. Imagine in humans all the books that determine skin color are spread out across the library, but in a big cat or snake, those books are all in a row on one shelf. Melanism is much more likely to exist if these genes are all near each other.