From Black Jaguars in Belize?: A survey of melanism in the jaguar, panthera onca by John R. Meyer:
There occurs in jaguars, as well as in leopards (Panthera pardus), a melanistic phase which has been much sought by zoos for exhibition purposes. This color phase was relatively difficult to acquire until the 1970's, by which time a few zoos, among them the Jacksonville Zoo, had begun to breed them regularly.
And likewise on the Wikipedia page for Black Panther:
This "black cougar" was most likely a margay or ocelot, which are under 18 kg (40 lb) in weight, live in trees, and do have melanistic phases.
I don't understand this term "melanistic phase", which makes it sound like the jaguar has a period of time in which it is black. However, it looks like it means something else, i.e., along the lines of What does phasing mean?, but I don't really understand that.
What does "melanistic phase" mean in regards to black jaguars?