The pentatonic scale is the precursor to the blues scale. The african-american slaves took the the pentatonic scale and added an extra note to it, known as the blues note the scale became the blues scale.
C minor penta......C Eb F G Bb C
C blues scale......C Eb F F# G Bb C
To create the deep, dark sound known as the blues, the slaves applied this scale over top of an all Dominant 1 4 5 progression (C7, F7, & G7).
A dominant chord is sort of a hybrid chord...half major, half minor. For a technical explanation the dominant chord is a major triad with an added minor 7th. So if we remove the 7th, or for a better explanation reduce the dominant chord to a triad we are left with the progression C, F, & G..the major 1 4 5 progression.
So...the 1 4 5 progression can be approached 2 ways, as a Major progression or as a Blues progression.