I doubt that Mezz's desire to be a black person is any different than those people who feel connections to places, times, cultures that are not their own, and try to live it out in some way by the clothes they wear, the books they read, the music they listen to, the films they see, the way they speak. I doubt Mezz was all that different than anyone else with a slightly fanciful nature.
Don't really know why Benny would've been considered a vulture by Mezzrow. He had tremendous respect for black musicians. (At least that's what I always heard.) Maybe because Benny became so famous? SUccess frequently breeds annoyance in other people.