When applying a guitar solo to a blues progression it is important to know what scale or scales will work and what will sound good. The most basic blues progression is constructed around I, IV, V progression in twelve bars. In the blues, these chords all tend to be the same type of chords, typically: seventh, major or minor chords. So, for example, a regular twelve bar blues will contain all seventh chords, a minor blues: all minor chords and a major blues all major chords.
Now if we examine a typical blues composed of seventh chords we will find that there isn’t one particular scale that contains all of the notes within these chords perfectly. The best match might be to play the mixolydian mode over each corresponding chord. This would mean mixolydian mode in the key of the I chord, played only over the I chord, mixolydian mode in the key of the IV chord, played only over the IV chord, mixolydian mode in the key of the V chord, played only over the V chord. This is one way to do it, your solo will certainly sound good over the chord changes and bright with the major 3rd.
When a blues progression is composed of entitely major chords, it is best to use the major scale, also known as the Ionian mode. The major pentatonic scale would also work well in this example. Both scales contain the notes which match these major chords perfectly.
A blues progression can be constructed with entirely minor chords. In this case use the natural minor scale to solo with. This scale will fit perfectly over this progression, because the chords contain the exact same notes as the corresponding scale.
What I would like to do now is explain the most common blues guitar scale and when it is best suited. This scale is simply called the blues scale and it is the minor pentatonic scale with a flat 5 added to it – R, flat 3, 4, flat 5, 5 and flat 7. Now remember I mentioned that the most common blues progression is constructed entirely with seventh chords(R,3, 5,flat7). This is the scale of choice to play over this type of progression. So you might be wondering why this scale has a flat 3 or minor 3rd in it – it doesn’t make musical sense using a minor 3rd over a major 3rd! Well, put simply this is what gives the blues such a distinctive sound – the flat 5 also gives this “bluesy” sound.
If you are looking to find a scale to solo over a typical blues progression, typically a I, IV, V chord progression, firstly look at the I chord. If it is a seventh chord, you could use the mixolydian mode over this chord and then use the corresponding mixolydian mode over the other chords. Maybe you could just use the blues scale over the entire progression, in the key of G, you would use the G blues scale. If it is a G major or G minor as the I chord you would use the G major and the G minor scale respectively. You could also just use the G blues scale over these two progressions as well, or a combination. As you can see, there are many options available to you when soloing over the blues. So have some fun, and enjoy these different sounding blues guitar scales.