People see black coyotes, and here is the kicker, they kill black coyotes and take pictures of them. Over in the predator hunting forum, a guy has game camera pictures of a black coyote.
The same cannot be said of "black" cougars, mountain lions, or "panthers". Plenty of people clain to see them, but hard proof is NON-EXISTANT.
As a matter of fact, I have never even seen a picture of a "dark pigmented" mountain lion. They are all pretty much the same, light brown/tan.