There is no question the very specific programs specifically targeted at improving the performance of under-represented minorities (read: largely “blacks” in the US) have had some effect. Every college and university takes great pains to solicit, nurture, graduate and place under-represented groups. On the employer side, every major corporation has a deep interest in recruiting and hiring minorities. None of this is evidence (as you seem to suggest I would have to conclude) of a core change in innate ability.
My maximum ability for the skillset of basketball is limited by my genes. Of course I would be better at basketball if, for the first period of my life I had been artificially kept off the court but in the second half I had been given the best possible nurturing for it. The fact that I improved would be a result of that nurturing; my maximum potential would be limited by my genes.
The change in employment success in STEM disciplines has not been accompanied by a commensurate change in quantitative testing (or for that matter, even much of a change in the number of PhDs awarded blacks in the STEM disciplines). It’s incorrect to infer an increase in demonstrated ability because of improved workplace diversity without accounting for differential hiring based on a desire to improve diversity. What needs to be shown is that, given equal educational opportunity, the STEM-related scores of blacks equal the STEM-related scores of whites and asians (or any other groups).
For all of the advanced testing for which I am aware (MCATS; LSATS; GREs…etc), the scoring gap remains rather wide. An employer may have other reasons than test scores to hire into STEM fields, and among those reasons is racial diversity (either voluntarily, or as a means to avoid criticism of being discriminatory). But that is a social-policy-driven success, and not a reflection of nurturing eliminating (or even significantly narrowing) a fundamental difference in ability.