The heptatonic, or seven-note, conception of the "blues scale" is as a diatonic scale (a major scale) with lowered third, fifth, and seventh degrees and blues practice is derived from the "conjunction of 'African scales' and the diatonic western scales".
Steven Smith argues that, "to assign blue notes to a 'blues scale' is a momentous mistake, then, after all, unless we alter the meaning of 'scale'". It is the 2nd mode of the Harmonic Minor scale.