[quote name='jakenewmanbass' timestamp='1386498416' post='2300616']
It stems from the voice... 'blue' notes are actually between semitones, so bending on the guitar is a natural way to achieve them.
On tempered instruments like piano and to a certain extent bass (as we tend to bend less) we play both major and minor third in quick succession to give the illusion of the note in between.
Try it, play C then Eb slide to E... then C again
Most blues will take major, dominant or minor sounds.
The dominant chords over which many blues sequences are played will handle a variety of notes that don't belong to the parent scale because they are in their nature less settled than major and some minor chords, they are traveling chords, there is a tension suggesting movement created by the relationship between the 3rd and the 7th wanting to resolve (up and down respectively), which is why you can play dominant chords ad infinitum; they will always lead you to another one or back to the one from which you came...
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I'm guessing this is an explanation of why, in my experience, when I play fretless or upright, hitting the 'correct' intonation seems to to be much more important on some notes than others, e.g. the octave needs to be correct, and the fifth, but the third can be out and just sound spicy.