You can also see it as a natural extension of the blues scale and blues harmonic context: essentially, blues harmony can be understood as dominant chords functioning as tonic chords, so in a minor blues you're seeing a harmonic context that implues 1-3-5-b7 over a chord that'd normally be 1-b3-b5. If you wanted to distill this tension down to a single chord, then a 7#9 is about as pure as you're going to get.
-D