The fact that indigenous north africans are somehow white, 100% caucasians, shows that the continent is very diverse,
Also what does being black means to people ? Someone can be very dark skinned and not from africa like some indians and indigenous australians. Also there are many group of people in Africa that don't look alike, like congolese ethiopians and south africans.
So you should have guessed it: The skin color of a group of people depends only of climatic and geographical parameters, so it's obvious that north africans located about 50 miles far from south europeans would not be radically different from them.