The only reason they would know is because it feeds the narrative of how racist America is. Point out to them that by refusing the vax they join the racists on the right side and you’ll begin to confuse everyone. Tell them its like voting for the Republicans. :)
1. Was it bad? Yup. Was it as bad as the Dims make to be? No. It didn't give anybody syphilis. It refrained from giving the penicillin that eventually (the study lasted 4 decades) was determined cured you of it. Bad? yes. They shouldn't have used those blacks as a control group to compare with blacks who got syphilis. Blacks today are led to believe the study actually dished out syphilis when it didn't.
2. Why do blacks today (it lasted 1932 to 1972) know about it? Because the Dims who at the time were pushing for segregation have spent the past few decades trying so hard to be on "the right side of history" by playing up "injustices" for blacks to forever be angry about. This includes perpetuating the lie that the Tuskegee experiment gave blacks syphilis.
3. And the Dims have the audacity to be surprised at blacks hearing their decades old fear mongering message and not wanting to play along with today's yet un-approved "vaccine".