They were never successful on the charts until Parliament was reinstated and the lines between the acts became more blurred musically with "One nation under a groove" and "Uncle Jam...". Sure, Funkadelic still didn't use horns but it could be safely argued they moved towards the middle.
Anyway, "Maggot Brain" and "Standing on the verge..." and those kinds of records are good examples of a black equivalent to Velvet Underground even if I don't think they were the total bomb vU was commercially (remember they were on an indie so I don't think bad sales mattered as much).

Shuggie Otis' album "Inspiration information" is another, better example and I could argue that acts like Terry Callier and Gil Scott-Heron are kind of in the same ballpark even if they weren't *that* unsuccessful.

Overall though, I agree but there have been some black acts that at least is somewhere in the VU/Stooges-category in terms of cred and longevity-status vs. initial sales...

BTW, the release of the VU debut was delayed with almost a year due to erve choosing to push Zappa instead as the weird act; had it came out in 66, I'm sure it would have done better; it was just SO wrong for 67 and the flowery "Summer of love"-thing, even the Warhol-connection went from a potential selling-point to irrelevant within one year. Nothing to do with the post, just saying...