According to this post [0] only allowing black people to post was a time limited action. As an Aprils fool joke only black people were allowed to post, which resulted in positive feedback from the community, according to the mods. Now everyone can post again, where as black people can get verified and a special flair (a small visual indication next to their username). Some threads [1] are reserved for verified people, but non-black person can also get verified (but might not get a flair).
[0] https://www.reddit.com/r/BlackPeopleTwitter/comments/b93w1j/...
[1] https://www.reddit.com/r/BlackPeopleTwitter/comments/gumxuy/...