Apparently Melanistic or all black jaguars occur due to a genetic mutation. This mutation causes the skin and fur to contain larger amounts of a dark pigment. These types of jaguars are found in rainforests because it is easier for them to blend into the dark shadows of the trees.
Interestingly, Black jaguars are often mistakenly lumped into the category of “black panther” with other melanistic big cats like the leopard, but they are a completely separate species. A great photo by @camwhitnall. What a beautiful cat!