No one was reporting sightings of black lynx? I'm not sure I've seen that claim. A google search in this case is pretty useless, as pretty much all the results are going to be recent.
But, as I said earlier, I'm not saying we have any melanistic cougars now. I'm saying it's very possible that melanistic cougars may have existed once (among other explanations for black panthers). Whether or not the eastern cougar once existed as a subspecies of mountain lion has been debated for quite some time, but the simple fact is we have very little data to say either way. It is very possible that Gloger's Rule applied to populations in the southeast and there were melanistic cats before they became extirpated a hundred years or so ago. There are not just historic accounts of sightings, but accounts of them being killed. The reality is we have no way of knowing what existed other than going off of historic accounts. We have areas in Texas where melanistic deer are much more common. It is certainly possible that the same was true of mountain lions in the south. Unfortunately they were extirpated along with their genetic footprint. The current populations of big cats being seen are primarily western migrants, which wouldn't have the same evolutionary pressure to preserve any melanistic mutations in their gene pool.
Of course, there are several other options for those historic accounts. Ocelots are known to have had populations extending into Arkansas and Louisiana historically. I believe fossils have been discovered in Florida. They are known to have a melanistic color morph. Bobcats are also known to have a melanistic color morph. One study in Florida encountered them at a rate of .5%. Either one of those could account for some of the stories of black panthers. Bobcats could also account for some of the sightings. But mostly it is probably a combination of a lot of things that lead to false sightings. People see what they want to see.
As far as the claim of no documentation of melanistic or black mountain lions goes, the vast majority of biologists are not in fact historians. They don't read primary documentation, they just repeat what other non-historian biologists have told them.