Yes, but you’ve also said that referring to people as “black” is dehumanising
No. Myself, and the commenter who was being responded to, said that referring to Black people as "blacks" is dehumanizing, which it is, and the context being asked for was an alternative term for an NSFW community. Hopefully that clarifies the issue.
Yes, generally referring to groups of people as pluralized adjectives is considered dehumanizing.
https://www.archives.gov/research/catalog/lcdrg/appendix/black-person
https://nabjonline.org/news-media-center/styleguide/#styleguidea
This is for the exact same reason you would not refer to a singular Black person as "a black". If you still have trouble perceiving the issue, consider how jarring the term "a gay" would seem in print.