This is vague and not good reasoning. You say "nothing wrong with black people" (p) --> "demographically represented in colleges" (q). In your attempt at proof-by-contradiction, you notice that NOT(q) is true, which should imply NOT(p)--something is wrong with black people. We know that's not right either, so what's wrong here is the initial p-->q. So you set up a new p-->q with "there is nothing wrong with admissions" --> "blacks are admitted in a demographical proportion", note NOT(q) again and conclude that NOT(p) is true, that is, there must be something wrong with admissions. Whether one wants to accept this new p-->q is completely separate from the issue of something being wrong with black people. Outside of this logic, NOT(q) could be a sign of any number of things besides "there's a problem in admissions."

Of course, the reality is that there are several feedback loops, and the actual causality doesn't flow one way, so it's hard to reason about (1-bit logic is insufficient) and even hard to influence. One might go around starting with blacks were once thought of as genetically inferior which led to some aspects of their current culture, and now that the genetically-inferior problem is taken away, the bad aspects of some black culture are still there, and they in turn go on to feed the continuation of the culture as well as produce subpar students and other bad individuals who will then be noticed by well-off folks who will associate the race with bad people, not because of some thought that they think black people are genetically inferior, just that they've noticed a lot of bad black people--of course, this leads to effects that are barely different from those caused by people actually thinking black people are genetically inferior, so you have "racism" where "racists" can be backed by observation (limited or not) instead of prejudice which will continue to feed into parts of the loop. And forcing employers to hire substandard black workers is not sufficient to fix the problem in the cycle of substandard black workers being produced in the first place, which has many sources like black culture, "racists", its members being on average poor since no one wants to hire them and other reasons, and more. So you try to fix the poor problem, but you don't because it's not as easy as just making sure they can't be refused a job just on their race. So you get employers complaining about the ineffectual overhead, and you spawn a new type of racist who assumes all black people with jobs were put there instead of a more qualified white man due to affirmative action, and these new types of racists start feeding into the cycle too.