There was actually debate about this in the black activist community in the mid-late 60s. The more common term was 'negro' then, you can see it in earlier MLK writings, League of Negro Voters, etc. Stokely Carmichael was a leading voice in advocating for 'black'.

"According to a 1968 Newsweek poll, more than two-thirds of black Americans still preferred Negro, but black had become the majority preference by 1974." https://www.ferris.edu/HTMLS/news/jimcrow/question/2010/octo...