There can be several explanations for the sightings. Hereare only a couple.

1. Light conditions. Cougars can easily appear very dark in low light.
2. Jaguarundi These animals look very much like a Cougar and are sometimes black. I doubt many of us could tell this animal from a Cougar they look so much alike. I didn't even know they existed until I started researching the black panther thing. I think this animal would account for many of the sightings in the south.
3. Belief. I think many people believe black panthers roam the US ( I know I did until recently) and they would be the scariest thing you would expect to encounter so I think a scary situation, belief, expectation, imagination, all may play a role.
4. Bobcats can be very dark or look very dark when seen unexpectedly. They would also appear larger under stressful situations.

Consider this: several people on this thread alone have either encountered a black panther themselves or know people who have. Take this sample across the country and you have literally thousands of sightings. There simply cannot be that many black panthers roaming the US even if they did exist. My feeling is that at least some of the factors above probably play a role in most of the sightings.

I read where Game Rangers say they receive thousands of reports of cougars/panthers across the country every year and most of the reportsare of black animals.