The Flynn effect is a gain in IQ resulting from improved nutrition among poor people. It is one of the few instances in which the environmentalists have been right about non-genetic influences on mental ability. The effect isn't one that goes far into prehistory—we note that Thomas Jefferson wasn't retarded and that Euclid was hardly a vegetable. Nor will it continue to increase IQ scores in the future. All (or nearly all) of the gains in IQ that can be made from improving nutrition have been made already.
As black IQs rose from 1945 to 1995, so did the IQs of whites. The environmental factors that bring up one race also bring up the other. The significant thing is that the gap remained between them during all of those fifty years.