See the report on humanist activism in Africa:
African Americans for Humanism in Africa by Norm R. Allen Jr.
Free Inquiry, June/July 2007, pp. 38-40.
Allen contrasts the notable activity in Africa with the scarcity of non-white participation in organized humanism in the United States. It seems to me an inquiry into the reasons for the depressing situation in the USA is in order. At the very least, one could attempt to enumerate the factors influencing black Americans' inclinations towards or against religion as individuals. Participation in organized atheist/freethought/humanist groups is another matter, but one could pose the question whether there is any socially causal factor to differentiate racial (non-)participation in these groups.
Another question is the existence of all-black atheist/freethought/humanist groups. I know there was a group in Harlem in the 1990s, unaffiliated as far as I know with any other. I don't know whether it still exists. I was invited to attend its monthly meetings, but I was never available in New York at the proper time. There is, however, a Harlem group now on the grid:
Harlem Community / Center for Inquiry
See also my web guide:
Black / African-American / African Atheism
One key question to pose is the degree of correlation of nonbelief among black Americans with racial integration. I would think that social segregation reinforces ingrained behavioral patterns, whereas interaction with diverse groups of people enables individuals to escape those patterns. But another question is the pattern of religiosity among the educated black middle class, and whether it is changing generationally. Other correlations to be factored in include regional and urban/suburban/rural factors, gender differentials, and behavior often deemed deviant in black communities (homosexuality, nerdiness, unusual cultural tastes, etc.).