There is no validity to this claim at all. The DNA heritage of North Africans has seen many contributions from invading peoples, but the substratum has stayed the same. Modern Coptic Egyptians, for instance, are people who look like the old Egyptians, and their Muslim neighbors still look like them.

Black people were well known on the southern shore of the Mediterranean since at least the time of Homer, but have not been a majority.

The idea that North Africa was "mostly Black" is an American Black nationalist fantasy.