There was a long period of time between when discrimination against blacks in employment was forbidden, and college prices rose to excessive levels. I doubt that signalling alone can explain the increase in college costs, or that letting employers discriminate based on race or IQ would reduce them. I'd blame it more on other government interference (such as subsidizing loans and making it essentially impossible to discharge loans in bankruptcy).

Furthermore, the situation of black people before the civil rights movement was bad enough that I'd be hard pressed to decide that even being massively in debt for a college loan is worse.