Well, blues are generally based on the I-IV and V ( 1, 4, 5 ) chords of any given key, and are usually all played as 7th chords. Unless it's a minor blues, then it is still the I-IV-V chords of a minor key, with the I and IV chords as minor 7ths, and the V chord as an altered ( adding a sharp or flat 5th or 9th ) dominant seventh. So in C major the chords would be C7, F7, G7. In a C Minor blues the most common treatment is going to be Cm7, Fm7, G#9 or Gb9.
Further, the most common scale to use over a blues is the minor pentatonic treating say the C Minor pentatonic as the scale of choice over the C7, F Minor Pentatonic over the F7, and G Minor Pentatonic over the G7. This is not the way I do it, nor is it the way Jerry usually approached a blues either. But, if you listen to SRV, or Jimmy Page, early Clapton, the Stones, or most of the original blues guys, this is the majority of what's going on. In short, blues really isn't the best place to try to apply theory because the reason blues sounds the way it does is largely due to the fact that it's theoretically "wrong", which gives it that edge.
Try this exercise. The construction of any Major scale is W-W-h-W-W-W-h steps ( with W being a whole step and h being a half step ). Play any of the Major scale patterns you have in your book starting on the root note, ascending and descending. So, if you were doing it in C, play C-D-E-F-G-A-B-C-B-A-G-F-E-D-C. Now, play the sequence starting on the next note, D-E-F-G-A-B-C-D-C-B-A-G-F-E-D. Then do it with E-F-G-A-B-C-D-E-D-C-B-A-G-F-E. Then F, then G, A, B. You have just played all the modes.
Now, if you have a way to record yourself or a friend, and you know some jazz style chord forms, record say a D minor7 chord over and over again. Now play the D-E-F-G-A-B-C-B-A-G-F-E-D sequence over it. You have just played the Dorian mode over it's home chord, a minor 7th. Next record say a G7 chord, and then play G-A-B-C-D-E-F-G-F-E-D-C-B-A-G over that. You have just played the mixolydian mode ( Jerry G's favorite, btw ) over a dominant 7th chord, which is where it belongs. If you keep doing this for each chord in a harmonized scale for all the different keys, you will definitely begin to hear the intervals and the different tonalities between Major and minor, and what fits where.
It's hard work, but what you want to do is develop your ear more than your sense of formulas to use, if you catch my drift. But the way to do that is to work the hell out of the formulas.
Out of the loop? I didn't know there was a loop!