The Black Panthers were not Black supremacists, although there were Black supremacists in the 60s. There are still Black supremacists today! No one cares much, because they have no power, and Black people don't listen to them. They mostly just spout nonsense at you while you're waiting for the bus. ??♂️
The FBI did not care much about Black supremacists in the 60s either.
What they feared most, was the rise of any "Black Messiah," who could lead Black people.