Black panthers don't exist. They're actually just big cats like jaguars and leopards who are melanistic (the opposite of being an albino). Where albinism deals with the lack of pigmentation, melanism deals with the dark colored-pigmentation of the skin. Check out 10 more beautiful examples of animals with melanism here.
Watch this video blog by Animalist to learn more about why the black panther is so often misidentified. Even though most of us will never see a 'black panther' in the wild, as they are extremely aloof and secretive creatures, we should protect their environment with additional conservation efforts. Check out this conservation volunteer program in the Amazon. Perhaps with all your efforts, you may one day even come across the majestic creature yourself.