Someone who don't know how to smoke pot in a bucket (pipe) gradually from side to side we say that he blacks it.
If you black, the weed won't taste as good as if it were green.
Mike: Hey James
James: Yeah?
Mike: Do you see that girl Kelechi
James: The black one?
Mike: Yeah.
James: What about her?
Mike: She's very intelligent, and has an amazing voice, and to top it all off she is absolutely beautiful.
James: What do you expect? She's black.