Genetically that doesn't seem to make sense. Perhaps it is really because a white mother is doing the child rearing, nurturing, and not a black mother?
If that sounds racist, I'm sorry; but I am simply curious as a scientist about 1) if this is really true and 2) what the cause really is.
About 50% of intelligence is environmental, however most of the genetic component of higher brain function comes from the mother. Nature uses female brains as the genetic baseline, and males for experiments. That's why female intelligence in general forms a narrow bell curve, while male intelligence has a wide bell curve with more men at both ends of the spectrum. Here are some articles:
Imprinted Genes Suggest your Cortex may derive from your Mother