The blues scale is extremely important in music like the major scale! However, most books tend to overlook it! It is usually disguised by it's close cousin - the pentatonic scale. But this is that scale, with one important note added....it is the note I have circled in each example! This is a moveable scale in any key depending on where you begin, the first examples are in A, and the last is showing how to play it starting from the 5th string in this example...the key of D. When I improvise, I incorporate a mixture of major scales and blues scales and alter in even further as I solo, which in turn develops into other various modes. The key is not so much to figure it all out, but to hear it so well, that you just go there and play beautiful lines in great taste....which requires a lot of practice...but all begins by playing these scales.