• To find the blues scale in any key, use this formula: keynote ~ flat 3-note ~ 4-note ~ flat 5-note ~ 5-note ~ flat 7-note
• The blues progression is usually played with three chords of the key: the I-chord, the IV-chord, and the V-chord. (The musical convention is to write chord numbers in Roman numerals—so 1, 4, and 5 become I, IV, and V.) In E-flat, which is what we’re playing here, that would be E-flat (I), A-flat IV), and B-flat (V).
• The blues scale of the key (here, E-flat,) will sound good played over ANY of those three chords. That makes the blues scale easy to use (and easy to overuse).