I think the claim is that prohibiting employers from asking about criminal records disadvantages black people precisely because black people commit more crimes.

I think the claim is that prohibiting employers from asking about criminal records disadvantages black people precisely because black people commit more crimes. If they're allowed to ask about criminal records, then the non-criminals, black or white, are all on an equal footing. If they're not allowed to ask, then they'll assume that the black applicant is more likely to be a criminal. [ I have no idea whether or not this claim is actually empirically true. ]