Blues scale

Blues scale can refer to any scale with added blue notes - flat third (b3), the augmented fourth (#4) or flat five (b5) , and minor seventh (b7), but usually refers to the six note minor pentatonic scale with an added flat 5.

The flat five rarely occured in interval earlier and classical music, and never in early church music, as it was deeed the diabulus in musica, a note that unresolved, sounded scary and would invoke the Devil.

 Pentatonic minor            Blues scale 
 |--1--|-----|-----|--4--|   |--1--|-----|-----|--4--| 
 |--1--|-----|-----|--4--|   |--1--|-----|-----|--4--| 
 |--1--|-----|--3--|-----|   |--1--|-----|--3--|--4--| 
 |--1--|-----|--3--|-----|   |--1--|-----|--3--|-----| 
 |--1--|-----|--3--|-----|   |--1--|--2--|--3--|-----| 
 |--1--|-----|-----|--4--|   |--1--|-----|-----|--4--| 
 Pentatonic minor            Blues scale 
 |--1--|-----|-----|--b3-|   |--1--|-----|-----|--b3-| 
 |--5--|-----|-----|--b7-|   |--5--|-----|-----|--b7-| 
 |--b3-|-----|--4--|-----|   |--b3-|-----|--4--|--b5-| 
 |--b7-|-----|--1--|-----|   |--b7-|-----|--1--|-----| 
 |--4--|-----|--5--|-----|   |--4--|--b5-|--5--|-----| 
 |--1--|-----|-----|--b3-|   |--1--|-----|-----|--b3-| 

The blues scale contains all of the blue notes.

Other scales named blues scales are the major blues scale that includs a flat third, also known as the “swing scale”.

 C   D   D#   E   G  A 
 1   2   b3   3   5  6 
   W   H    H   W   W