The blues is typically not in a single key but oscillates between three "keys". In fact it also oscillates between major and minor feeling of the base key.
The task is to find time intervals for each of the "keys". If the song has the 12-bar values form then we could find the whole 12-bars blocks.
The algorithm can utilize some prior knowledge of blues and some more ridig structure than for general music.
Segmented blues can be then utilized for further experiments.