I have just released a 2-disc album called Major Betty Blues Backing Tracks. It contains 12 bar blues backing tracks in all the major keys, at both 90 bpm and 110 bpm.
They are great for practice and improvisation and a fun way to learn the 1, 4, and 5 chords in all the major keys. You can also practice/improvise on the scales and build your confidence and ability to voice chords in all the inversions.
A sample track has been uploaded to the music page on this site.