yeah, you’re right but they’re two different cases. notice how when it’s right you don’t pluralize it with an -s because some adjectives have a form of a plural noun, so they don’t have a singular form: “a poor” or “a black” is just yikes. you can find words like “rich” as plural nouns apart from the adjective forms in the dictionary. you might find “female” and “black” as a noun for people too, but they should be marked offensive either directly or in usage notes.

so that’s the distinction. “black” or “female” don’t exist as plural nouns like “the rich” or “the blessed”.

interestingly enough there are exceptions. there is no plural noun “the gay” but “gays” usually isn’t offensive as a noun, but also “a gay” is weird and offensive. language is complicated.