The term "black panther" is just a generic term for any large cat with a black coat.
The black coat comes from a condition caused by the agouti gene, which regulates the black pigment in the hair shaft. This gene produces an excess of melanin, and animals with a surplus of melanin are called "melanistic". This condition is most common in Asian and African leopards, and South American jaguars.
The interesting thing about "black panthers" is that they aren't necessarily completely black. Many of them still have their spots, but they are hidden by the excess black pigment. When these animals are in direct sunlight, their patterns are more visible. This is called "ghost striping".
Melanism in leopards is caused by a recessive gene mutation. One litter can produce both melanistic and non-melanstic litter mates. Black leopards are more common in the equatorial and tropical rainforest. In parts of southern India, the black leopard may be more common than the usual spotted leopard. Black leopards are the most common type of black panther in captivity, although they are reported to be less fertile than common spotted leopards.
Melanism in jaguars is caused by a dominate gene mutation. This means that a pair of black jaguars can produce either spotted or black cubs, but a pair of spotted jaguars can only reproduce spotted cubs.
There has never been a documented case of a black cougar, or North American Puma. Truly melanistic cougars have never been photographed, killed, or bred in captivity. The melanism that is a genetic variation just is not part of a cougar's genetic make-up. Unconfirmed sightings have generally been attributed to errors in species identification. These sightings were probably either a margay or ocelot, both of which can have melanistic tendencies but are much smaller than the American cougar.
Another explanation for the numerous reports of people who claim they saw a black cougar could simply be low lighting and shadows cast during the early dawn hours and late evening hours. Cougars are the largest cat in the United States, and the most studied. If there were a black cougar living in the wild, scientists would have known it by now.
The next time you are at a cocktail party and the dinner conversation turns to black panthers, you now know that the politically correct term is either black leopard or black jaguar. In addition, they are not endangered because there is no species of big cat called "Black Panthers."