Why is the American conception of race in Africa useless?Chapter 19 How Africa became black. Thank you

I think that the answer that you are looking for is to be
found on the second page of the chapter -- p. 377 in my copy.  What Diamond is saying is
that we tend to think of Africans as a bunch of homogeneous black people when in fact
there are many different kinds of people in Africa.

Diamond
argues that whites (up in the north) and blacks were both native to Africa.  Also,
however, he points out that there are two other groups of continental Africans that are
not physically similar to blacks.  These are the pygmies and the Khoisan.  Finally, he
says that there are Indonesians who are native to the island of
Madagascar.

Because of all this diversity, it is incorrect
for us to think of Africans as all being black (and that's without even talking about
the diversity of people who we would call black).