You’re claiming then that if the welfare payments stopped the unmarried black women would suddenly clamp their knees together and black men would wake up in the morning, start working on their business plans, and buy suits and ties in preparation for making their presentations to the venture capitalists?
Yes, in time.
It would not be that dramatic a change, but behavior would change over time, in alignment with the new incentives and disincentives.
A woman who would have to support her kids herself, by the sweat of her own brow working in McDonald's, would develop more enthusiasm for birth control, and would have decreased interest in jobless boyfriends.
Guys who had difficulty in getting laid, because being unemployed made them less attractive to women, would have an incentive to get a job, and in getting the education that would allow them to get a better job.