The harmonic scale is used for developing harmony (chords). The melodic minor is used in melodies. They're rules with how to use the scale, like the difference between the ascending melodic and descending. To me the blues scale is just a minor pent with a b5. The b5 is usually a leading tone to the 5th or dominant. Great for melodies cause blues is build off dominant chords. Musically there are no rules on how to use a scale, except play what sounds good.