Blues is typically identified by its twelve bar format. In many blues songs, the structure of the song is such that the keys change over 12 measures and then return to the initial state. Listen to BB King's The Thrill is Gone (which was the subject of a rap remake by Hammer) to get a clear presentation of the twelve bar structure in action. Interestingly, in general, songwriters in blues and R&B tend to register with BMI instead of the larger ASCAP.