I think I see what you're saying so let me try and jump in here. In a blues form, you are playing all 7th chords, the I7, the IV7, and the V7. It's not a diatonic form. If you want to solo using the harmonic minor or any of its modes (the fifth is the only widely used one) then you should be playing over chords derived from the harmonic minor scale. There are some chords in this scale that cannot be derived from the major scale. There's a Minor/Major 7 chord on the root, and an augmented chord too. I don't feel like listing them, but if u sit and stack thirds you can figure it out. Is that what your question was?