Its nothing more than historical revisionism. The Nubian dynasty only came to power near the end of classical Egypt, when the empire was already in decline. The Nubians couldn't run Egypt properly, and they were conquered by the Assyrians and then the Persians.

There was no such thing as black Egyptians, this is a fantasy concoction of blacks who never had a successful state to call their own. The Nubians were outsiders who came to power through conquest at a time when Egypt was weak, and they ran the empire into the ground with ill-advised wars.