I play lots of blues guitar, I am familiar. That chart is basing it off the C minor scale which would correspond to the Eb major diatonic scale.

If you play an Am blues scale which is, you'll see how the blue note is really just one note (it's a pentatonic scale with note between the 4th and 5th added): it would be A C D (D#/Eb) E G A. That D# is the blue note.