BTW - when playing the blues, two scales are used, primarily - the blues scale and the mixolydian scale.

The blues scale stays constant throughout the whole 12-bar blues, but the mixolydian scale needs to change when the chord changes.

ex. - if A7 is the I, then you would use A blues scale, and A mixolydian

but on D7 - use A blues scale, but D mixolydian.

You can find the mixolydian scale in my modes tutorial on my instructor's page. (link on my signature).