So half of the mystery has been solved.
West African immigrants do better than indigenous US African-Americans because of immigration selection. In her exhaustive analysis, Suzanne Model shows that the relative West African success can be explained in total by selection and, moreover, that West Africans do no better than internally immigrating African Americans. From a condensed version of the book :
To review, West Indian immigrants have long fared better economically than African Americans. This generalization holds even when immigrants and natives are assigned the same age, education, location, etc. Experts have proposed four distinct explanations for this state of affairs: West Indians are positively selected immigrants, Caribbean slavery taught West Indians valuable skills, socialization in an all-black society is psychologically beneficial for blacks, and white Americans discriminate less against West Indians than African Americans. When the four explanations are tested empirically, only positive selection receives support. This is not to say that growing up in an all-black society might not provide psychological benefits or that whites might not respond positively to blacks with a Caribbean accent. But even if these relationships hold (which has yet to be demonstrated), there is no empirical evidence that they enhance West Indian economic attainment. Rather, West Indian success can be attributed entirely to the greater talent and ambition of those who choose to move. Similarly, the subset of African Americans who are voluntary internal migrants are better off than their less venturesome counterparts. Once this point is clear, it is easy to see why West Indian success offers no lessons for African American improvement.
So, self selected African immigrants have a relatively higher IQ and take advantage of racial quotas. No surprises there. This leaves the other half of the mystery. Why do the offspring of West Indian Black immigrants — unlike those of some other immigrants — regress socioeconomically …
…and perhaps otherwise?
 Model, 2008. The Secret of West Indian Success
 Sakamoto, et al., 2010. Does an Immigrant Background Ameliorate Racial Disadvantage? The Socioeconomic Attainments of Second‐Generation African Americans.