My notion is that the #9 chord (aka the 'Jimi Hendrix' chord or the 'major-minor chord'--both silly names) is the essence of Blues Harmony in the vertical sense. Considering, say, a G root, it is the Gmaj chord, the Gmin chord and the G7 chord all wrapped up into one tight little bundle. Now perhaps you can begin to see why I say that the Blues "blends" keys rather than merely interacting them. Why? Because, say, Gmaj is from one key, Gmin is from another key and G7 is from yet another key. And the G7#9 chord is a blend of all three chords just mentioned.
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https://www.youtube.com/watch?v=UrSze1geudc