Black people in the early twentieth century attempted to play the music of their native continent on Western instruments. These instruments could not completely replicate the notes used in African music, as African music uses a non-equal tempered tonal system. In other words, African music uses tones in between the notes found in Western music. Therefore, different scales evolved from black people attempting to approximate the African tones on Western instruments, giving us what we know as "the blues."
Blues scales are based on five primary tones. These tones make up what is known as the "pentatonic scale."
One such standard blues scale consists of the root, third, fourth, fifth and seventh intervals of a seven-tone scale. For example, in the key of C major, those notes would be C, E, F, G and B.
Using a major pentatonic scale creates more of a "happy" sound. But the blues are famous for that "sad" sound. Thus a minor pentatonic scale is used as the basis.
In a minor pentatonic scale, the third and seventh interval are diminished, or flatted by lowering the note a half step. Thus, in the key of C minor, those notes would be C, E flat, F, G and B flat.
The diminished tones are called "blue notes," added to give a blues scale what is usually described as a sense of "bluesiness." Another blue note---a diminished fifth---is added to the minor pentatonic scale, giving us a whole, six-tone blues scale. Thus, in the key of C minor, this blues scale would be: C, E flat, F, G flat, G and B flat.
Of course, you may wish to play the blues in a major key. There is a major blues scale that can be employed using a different pentatonic scale. In the key of C, we would use the root, second, third, fifth and sixth intervals. To make the scales "bluesy," we add a blue note---a diminished third. Thus, we have the C major blues scale consisting of C, D, E flat, E, G and A.
All major scales have a relative minor. In the case of C major, its relative minor scale is A minor. To create an A minor blues scale, we would follow the pattern of root, diminished third, fourth, diminished fifth, fifth and diminished seventh. Thus the notes would be A, C, D, E flat, E, G and A.
Notice that if you start on the second interval of the A minor blues scale, you end up playing a C major blues scale. In other words, by learning one blues scale, either major or minor, you have actually learned two basic blues scales.
To play a major blues scale based on a minor scale, start on the second interval. To play a minor blues scale based on the major scale, start on the sixth interval.
If you're learning how to play blues scales on the guitar, you will have an easier time learning multiple blues scales because the same fingering pattern is used no matter where you are on the fretboard. For example, the finger pattern you use to play the notes of a C minor blues scale can be used to play an F minor blues scale. Simply move to the fret you'll use to play F as the root, and use the same finger pattern to play the diminished third, fourth, diminished fifth, fifth and diminished seventh.