What makes you think this is obvious? While racial IQ differences certainly aren't ruled out a priori (Ashkenazi Jews are the quintessential example), Occamian reasoning about the black/white divide doesn't indicate that genetics is part of the best and most parsimonious explanation. There are adequate other factors at work - you can pick up a lot of data from studies on things like stereotype threat, for instance. And the fact that biracial children do better on IQ when the mother is the white parent than when the mother is black seems strong evidence to me that genetics are not the whole story, if they play any part at all.