Both Sides Lost
According to Chanda Chisala:
The fact that black immigrants to the United States have shown achievements that are superior to native black Americans has been a phenomenon studied since at least the 1970′s. … What most of these theories failed to predict was that the children of these immigrants would also show exceptional achievements, especially academically. It is only in recent years, as the immigrants have stayed long enough to produce a sufficiently high number of offspring, that it has been observed that they are over-represented among high academic achievers, especially when compared to native blacks, particularly at very elite institutions.This disproves the favorite theories of both the Left (that the black–white academic gap is due to the effects of prejudice) and the hereditarian wing of the Right (that the black–white academic gap is due to the effects of genes). The children of African immigrants have the same genes and face the same prejudices as the descendants of 18th-century kidnap victims. In other words, both sides have lost.
As for what is causing the black–white academic gap … stereotype threat might be part of the explanation. I suspect the affirmative action is aggravating that.