How was Sheldon a victim of racism?

Sheldon was unable to find work, not because he wasn't was talented musician, but because he was black.

He was a polished jazz player, whose poverty had less to do with his musical ability and more to do with his color. Henry had liked him immediately. Not because they were both outcasts, although if he really thought about it, that might have had a ring of truth to it—no, he liked him because of his music.