when plural you want to use color descriptors as adjectives. black people, not "blacks", white people, not "whites".
nobody can say why, but they are not synonyms to be used interchangeably between sentences, and will undermine your positive intent.
Why is this preferred and how would I know it's preferred (absent this conversation)? Is it a well known faux pas or something one person on the internet is nagging about (no offense)? In my mind, "people" is inferred. I think this is how people talk where I live, pretty sure I hear it on the local news/media. When I think of terms like "black college" it's inferred to be a college that is predominately attended by black people. The college is not literally the color black like with phrases like "white house".
Reminds me of when I was a kid and my grandpa would use the word "oriental" to describe Chinese, Korean, Japanese. He would use worse words at times but this was his version of the politically correct vocabulary. It was very commonly used as a PC phrase. As I got older, I learned it wasn't really the best word and reasons why. But it never would have been obvious to me that it was a racist term and seen as negative to those it was meant to described.
I am not sure there is any etymology to follow here, aside from the common theme of being recognized as people.
Regarding consensus, you can try blackpeopletwitter reddit on a throwaway. you can look at comment threads and see the reactions and facepalms when people use the adjective as a plural noun.
People definitely say it, but not people considered aware.