Chicago Blues has a more extended palette of notes than the standard six-note blues scale; often, notes from the major scale are added, which gives the music a more "jazz feel" whilst still being in the confines of the blues genre......
Another notable point is that Chicago blues contains many dominant 9th chords, and the scales usually contain 9th notes. Note that a 2nd is the same as a 9th (the notoriously confusing '9=2' seen on many chord charts reflects this fact), and this is especially emphasized on guitar, on which it is much more difficult to span octaves, than on a piano.
Wikpedia
Nå det er derfor.....

Mange mange gæster (blandt andet Rick Holsmtrom) og så denne skæve lyd - som om de ikke tager det alt for alvorligt, mens de leger med the blues....
Zig