Yeah I know ethnically that Africa is very diverse genetically. But racially, a typical man from Zambia is racially the same as a typical Nigerian.
Race:
a. any of the traditional divisions of humankind, the commonest being the Caucasian, Mongoloid, and Negro, characterized by supposedly distinctive and universal physical characteristics: no longer in technical use.
b. an arbitrary classification of modern humans, sometimes, especially formerly, based on any or a combination of various physical characteristics, as skin color, facial form, or eye shape, and now frequently based on such genetic markers as blood groups.
c. a human population partially isolated reproductively from other populations, whose members share a greater degree of physical and genetic similarity with one another than with other humans.
http://dictionary.reference.com/browse/race
So using the term black is a social, political AND scientific term.