In Guns, Germs, and Steel,
The answer to this can be found in Chapter 19. Specifically, it can be found on p. 386 of the paperback edition of the book. On this page, and on succeeding pages, Diamond tells us that agriculture in Africa began in the north. This might be contrary to some people’s expectations since the north of Africa does not seem as fertile to most people as sub-Saharan Africa does.
One reason why North Africa was the first to get agriculture is that some parts of that region are similar to the Fertile Crescent in terms of climate. Some of North Africa has a Mediterranean climate just like the Fertile Crescent does. That means that plants from the Fertile Crescent could relatively easily be adopted for use in North Africa.
This is all very important to Diamond’s explanation of why Africa “became black.” He tells us that agriculture developed north of the equator in Africa. This is also where the speakers of the Niger-Congo languages (the people that Diamond calls “black”) came from. Because they got agriculture first, they were able to dominate the people of Southern Africa and that region “became black.”