These black students are simply distributed differently within both systems -- no longer being mismatched with institutions whose standards they don't meet. They now have a better chance of graduating.
What of the idea that affirmative action has helped blacks rise out of poverty and is needed to continue that rise? A far higher proportion of blacks in poverty rose out of poverty in the 20 years between 1940 and 1960 -- that is, before any major federal civil rights legislation -- than in the more than 40 years since then. This trend continued in the 1960s, at a slower pace. The decade of the 1970s -- the first affirmative action decade -- saw virtually no change in the poverty rate among blacks.
In other words, most blacks lifted themselves out of poverty but liberal politicians and black "leaders" have claimed credit. One side effect is that many whites wonder why blacks cannot lift themselves out of poverty like other groups, when that is in fact what most blacks have done.
Affirmative action is great for black millionaires but it has done little or nothing for most people in the ghetto. Most minority business owners who get preferences in government contracts have net worths of more than one million dollars.
One of the big barriers to any rational discussion of affirmative action is that many of those who are for or against it are for or against the theory or the rationales behind group preferences and quotas. As for facts, the defenders simply lie.