Why are there so few famous black rock musicians after Jimi Hendrix? Had African American pop culture already moved past rock to funk and Motown?

I think there are some misconceptions and/or unrealised assumptions inherent in your question.

Firstly, you likely have an implicit definition of 'rock' in your mind, which likely includes Chuck Berry and Little Richard, but which likely doesn't include Stevie Wonder and Rick James. In the typical rock history that's told in magazines like Rolling Stone, Chuck Berry and Little Richard are included as part of rock history, but Stevie Wonder and Rick James typically are not - they're seen as part of a separate story.

However, almost all of the stereotypical rock bands that we think of when we think of 'rock' - the Rolling Stones, or Led Zeppelin, for example - have more than a little 1960s soul and funk in their sound. I mean, the Rolling Stones in the 1970s covered two Temptations songs, reasonably faithfully, and had black backing vocalists and Stax-style horns. Led Zeppelin's 'Trampled Underfoot' is as obviously influenced by Stevie Wonder's 'Superstition' as 'Shining Star' by Earth Wind and Fire is. If African-American pop culture hadn't moved past Motown, neither had the Rolling Stones or Zeppelin.

So in many musical respects, it's not entirely clear where the boundaries between rock and soul or r&b are, as there's clearly a bunch of intermingling between them. Funkadelic are very influenced by Jimi Hendrix, for example, and I strongly suspect that if Jimi Hendrix were still alive in 1976, he would be jamming onstage with Bootsy Collins and George Clinton (perhaps on the track 'Who Says A Funk Band Can't Play Rock?'?). But despite being so strongly influenced by Hendrix and despite actively claiming that they play 'rock' in song titles, Funkadelic are not usually included in the list of bands that get called 'rock' - after all, George Clinton and P-Funk were making dance music that was very popular amongst black audiences.

Perhaps the difference between 'rock' and 'r&b', then, is primarily a difference in musical philosophy. Elijah Wald argues in the book How The Beatles Destroyed Rock'n'Roll that the 'rock'n'roll' of the 1950s and the 'rock' of the mid-1960s onwards are entirely different genres with very different aims and musical characteristics (thus the book title - the Beatles killed rock'n'roll by turning it into something quite different).

There's a set of assumptions in most 'rock' that John Covach describes as 'the hippie aesthetic' - a belief in individuality and personal freedom, a Romantic view of the nature of art, a fetishisation of the electric guitar, a certain anti-establishment attitude, a recognition of the importance of writing your own songs, etc. It's just that the people who made 'rock' - including Jimi Hendrix - listened to 'rock'n'roll' when they were teens, and used its tropes as a starting point, so people telling the story of 'rock' feel obliged to start with 'rock'n'roll'. In contrast, 'rock'n'roll' was just raucous dance music aimed at teens.

This hippie aesthetic, by and large, best suits middle-class members of the dominant ethnicity in a society. After all, individuality and anti-establishment attitude are seen as much more confronting coming from people who don't share the upbringing and skin colour of the establishment. Where black musicians do share something of the hippie aesthetic - Stevie Wonder on Innervisions or Marvin Gaye on What's Going On, for example - they usually have to come across as non-threatening to be accepted, and so they rarely share the loud guitars and confrontational attitude that goes along with 'rock' (and even so, Wonder and Gaye both received pushback from their record company for getting political).

In 1992, when the rapper Ice-T started a heavy metal band called Body Count and released an album with a song called 'Cop Killer' on it, political pressure very quickly caused the record company to remove the album from shelves (I doubt there was much political pressure about Eric Clapton's cover of 'I Shot The Sheriff'). I guess, what with Ice-T eventually becoming a Law And Order cast member, he eventually decided that if you can't beat them, join them.

Of course, people growing up in African-American communities don't often get the same exposure to rock music as a white kid living in suburbia - it's simply not marketed to them in the same way, and it's rarely the dominant style of music their friends would be listening to, etc. So I would also say that most of the black musicians who do get called 'rock musicians' -including Hendrix - very often grew up in situations where they had stronger engagement with mainstream white culture than the average. And while they're reflexively called 'black', most of these musicians are biracial in important ways. So Prince grew up in Minneapolis, and Hendrix grew up in Seattle, neither of which are renowned for their sizeable black communities; and both musicians have some African ancestry but are biracial to some extent. Lenny Kravitz's dad is Jewish. Michael Jackson's engagement with mainstream white culture very famously extended to his choice of skin colour.