So basically, because their families were enslaved for generations and they only had very limited, specific skills to do slave-like jobs, once freed they had a single option for employment and had to return to their old life, now with a small amount of pay.
OR, perhaps if they weren’t enslaved in the first place, they might have had an opportunity for an actual education or acquisition of skills that would lead to more diverse employment opportunities.
If people of African decent were running the majority of corporations and swimming in wealth in the 20th century, this might have a lick of validity to it, but the fact that the opposite happened is enough to put your argument to bed.