Phil,
the specific study showed a large improvement in black students and no improvement in white students; it was performed on both. It's a recent result and I agree it needs to be repeated. (more details:
http://psych.colorado.edu/~gcohen/achievement-gap.html)
If we run the Flynn effect back to the 18th century in Europe and the Us then we get scores on modern IQ tests of 70-75 - roughly what we see in sub-Saharan Africa. Given that they're effectively living in the 14th century that's not surprising.
So, between those factors and a known culture among black Americans that discourages achievement as 'acting white', we would expect to see the black/white differences that you refer to, whether or not there's actually a genetic difference. Therefore I don't see a need to invoke a genetic explanation.
Also, there are pretty significant mean IQ differences between nations that should be relatively similar in genetic makeup. Take Germany and France; some reported scores are 107 vs 98 (
http://en.wikipedia.org/wiki/IQ_and_...lth_of_Nations), 109 vs 96 (
http://www.iqcomparisonsite.com/NationalIQs.aspx) and 107 vs 94 (
http://www.timesonline.co.uk/tol/new...icle697134.ece). Average difference just under 12 points.