This construct is used to delink Africans from, say, Egypt or Carthage - even though the inhabitants would very well have been called black.
It is also used to reinforce the invasion of South Africa, Zimbabwe etc - if there is a Black Africa, there must be a white one, hence legitimacy. The idea of a Black Africa legitimizes invasion, of both Europeans and Arabs, suggesting that these populations have their "homes" on the continent, when in fact they dont.
Africa is BLACK, period. Of course this is social blackness. To speak of a special corner of Africa are being black, as opposed to the rest is risible. We only do so when we want to legitimize the Arab and European invaders and interlopers.