For six years husky-voiced Millie Jackson has been turning out R-rated "raps" that are banned from radio airplay but still manage to reach the charts and a large following. Small wonder. Though her lyrics are sexually explicit (and, on This Is Where I Came In, mildly violent), Jackson proves herself a maestro of energetic blues. Her version of This Is It is a hard-driving comment on a relationship gone sour. Though not to everyone's taste, Milie is a powerful performer.