That's not the issue at all.
The percentage of people in the general population (black or otherwise) with PHDs is so small that it barely affects unemployment. For example, even if black people got PHDs at twice the rate of white people, it still would only have a small effect on overall blck unemployment rates. And you're living in a dream world if you think there ever will be some point where half of black people (or any other group) get PHDs. Not to mention the huge amount of debt that you'd have to take on and the possibility that it still might not lead to a high paying job.
The issue is that a large percentage of the black population is lacking in marketable job skills, along with a huge amount of racism in the job market (for example, white high school dropouts getting preference over black college grads). Focusing specifically on PHDs is a waste of time.
This is basic common sense but its futile trying to beat that into the brains of some of these dudes, man.
The solutions are much larger than "go into STEM" and go back to capitalism itself.