Supposedly, education for blacks in the South was awful prior to the civil rights movement. If education makes any difference, perhaps IQ scores of southern blacks have gradually improved since the 1960s. The graph below shows mean IQ per decade for southern (blue line) and northern (red line) blacks (N = 4,435):
Notice, too, how the means increased for both groups, especially for southerners, over this period. Perhaps southern schools have gotten better, and some other factor has boosted scores for blacks in both regions. It could be some biological factor like nutrition, or perhaps something like more exposure to mainstream vocabulary words via television. I'm pretty sure the same 10 questions have been used since the beginning, so the quiz hasn't gotten easier.
Here is the graph for whites (N = 24,363):
It looks like northern blacks and especially southerners of both races benefited from something.
UPDATE: The South has gotten to be a more attractive region post-civil rights era. Perhaps part of the story is intelligent black and white northerners moving south, thus boosting the mean IQ.