Blues in Bb...I may just way Bb or Eb or whatever. I mostly leave the extensions to you. The fifth bar will be an Eb7 in this case. Lot's of times that will be followed by an E diminished in the next bar and a Bb again in the bar after that and a G7 in the bar after that. Instead of the E diminished, try an E half diminished (m7b5) for two beats and an A 7 for the last two beats in that measure. Then play a Dm7 or a Dm7b5 in the next measure... bringing us to the G7 in the following measure. we start with a simple: Bb |Eb | Bb | Bb | Eb |Edim | Bb | G7 | Cm | F7 |Dm7, G7| Cm7, F7 | and the few simple substitutions: Bb |Eb, Edim | Bb | Bb7 | Eb |Em7b5, A7 | Dm7 | G7 | Cm7 | F7 |Dm7, G7| Cm7, F7 | The Em7b5 to A7 functions the same way the E diminished does and can also be used in the second bar. If you want to get carried away, you could even try this next bit. (I'll flesh out the rest a little more on this one too) Where you see two chords in a measure,they get two beats each. If there are four chords in a measure, they get a beat each: | Bb | Em7b5, A7 | Bb | Fm9, Bb7b5 | | Eb9 | Em7b5, A7 | Dm7, Dm7b5/Ab | G7+5, G7 | | Dbm9, Ab7+5b9 | Cm9, G7+5b9 |(bars 11 and 12 on next line) | Em7b5, A7, Dm7b5, G7+5 | Dbm9, Ab7+5b9,Cm9, G7+5b9 | This is one of the many ways we can augment the blues progression to give us a few more changes to work with that we don't usually get to see in a blues. There are a million possible variations and they can all be pretty fun. You can use the just parts that you like, too. You don't have to use the whole prog.