In our basic G blues progression, this was what phrase 2 (bars 5-8) looked like. Two bars of the IV7 chord C7, followed by two bars of G7.
The first substitution is one we used in bar 2 of phrase 1, replacing the C7 with C#dim. Because diminished chords repeat in intervals of a minor third (every three frets), we could also use Edim, Gdim, or Bbdim.
Another option for bar 6 is to use a tritone substitution and replace the C7 with an F#7. We can also use the same minor chord principle we explored in bar 4 of phrase 1 by adding the C#m7 chord before the F#7.
How does the tritone substitution work? Compare the following two chords.
As we can see, they share two notes, B and F. B is the 3rd of G7 and the b7 of Db7. F is the 3rd of Db7 and the b7 of G7. In any 7th type chord (Dominant7, major 7, minor 7, minor 7b5), the 3rd and 7th are the two most important notes in the chord as they define the chord quality (the chord type). So substituting the Db7 for G7 still gives us the important notes of the chord, plus the b5 (Db) and b9 (Ab) alterations.
In our bar 6 example, the C7 and F#7 share the notes E and Bb. Replacing the C7 with the F#7 is like playing a C7 with no root and the alterations b5 (or #11) and b9.
Because bar 9 is going to be changed to an Am7 chord, we are going to anticipate that change in bar 8 by going to the vi7 chord (E7), the V chord of the Am7. Adding the Bm7b5 gives us a minor ii-V-I progression. The b9 is a common alteration for the V7 chord in this progression.
Another variation here is to add the C7 chord to beat 3 of bar 7, giving us a I7-IV7-iim7b5-VI7 progression for bars 7 and 8.
Another progression commonly used in bars 7 and 8 is to resolve to bar 9 chromatically.