I'm with Rich Cronk on this one. I'll bet that back there somewhere somebody saw a black cougar to get these stories started. I saw a "black" one in Washington State coming to a call, a silhouette against a background of snow on an overcast day. Had we shot it I assume it would have been the usual grey/tans/reds with maybe some black trim, or maybe a darkish variant of those.
There may be more black genes in some geographical gene pools, such as back east. For example, I've seen half a dozen white or partially white deer, all of them blacktails. All but one came from two counties in Washington State, most of them from an area 30 miles across. Within Washington and B.C. I've never seen anything but a black coloured black bear west of the Cascades, yet east of the Cascades they come in a rainbow of colours. There are lots of reds or cinnamon, some chocolates, almond white and a blue looking "glacier" colored one once in awhile, or even combos of those such as a blonde one with cinnamon or black legs and feet.
Black cougars must be super rare however or we'd come up with the body or pelt of one more often than we have. I've seen one all black elk, in Idaho, one white grizzly, and I mean snow white, in B.C., a black grizzly also in B.C.. I've seen a jet black possum in California (beautiful pelt)and a few other odd coloured critters over a lot of years of outdoor looking. Black "panther"? Possible, yes, but it must be an anomaly rather than breeding true.