"The Blues Scale"
A diatonic major scale incorporating a lowered or bent 3rd, a lowered or bent 7th and sometimes a lowered or bent 5th to approximate melodic notes that originated in African work songs.
Since the actual pitch is unavailable on a mandolin, the lowered note is often "pulled off" against the natural pitch to approximate the blue note, by placing the finger on the note and quickly pulling it off after pick strike.