Some ethnic groups do have different average levels of intelligence. Ashkenazi jews, for example:

http://en.wikipedia.org/wiki/Ashkenazi_Jewish_intelligence

My own guess is that for some blacks, there has not been enough time for the Flynn Effect to occur for them to catch up with the general population. This is not due to any fault in them, but a result of no/substandard education for many generations. I imagine in a few generations any difference from the average will disappear. http://en.wikipedia.org/wiki/Flynn_effect