Watch the Blues Scale online guitar lesson by Andrew Ford from Blues Bass Survival Guide
The blues scale is a lot like the minor pentatonic scale. Only difference is that it adds one additional note, which is the #4 or maybe more accurately labeled the b5. This is what we would call one of the blue notes in the scale, the others are the minor third and b7. This scale is often used on a dominant 7th chord or a minor chord. It is also used as a tool for beginning soloist so they can use one scale and solo through a whole 12 bar progression. Here is the A blues scale, you should learn this scale in all 12 keys. In addition, I have a one bar riff based on the A blues scale that shows one way to apply it. A couple of slurs are used also to make it more melodic.