Even if you're brand new to jamming on the Blues it doesn't take long to notice that in Blues, just because a scale tone fits well with one chord doesn’t mean that it will necessarily fit well with the others...
Basic blues revolves around three primary chords. There are sometimes others, but for now let’s consider the primary three as the core of the blues universe. They are known as the "I-IV-V," Blues progression chords, and they are normally performed as Dominant 7th.
Have a look at the progression shown below. It is what we commonly refer to as the "12-Bar Blues Progression." This one is in the key of "C Blues."
Example 1). click on any image below to enlarge...
In a majority of cases, the typical guitarist would play the "C Blues Scale" over all the chord changes in example one. And, while that would sound "OK," it doesn't truly highlight the sound of each chord.
For instance, the B that is found in the G7 chord isn't in the "C Blues Scale." And, if we added it in, it's sound is actually completely inappropriate for the C7 chord, because the C7 chord has a B♭, and these tones conflict.
So knowing the chord tones helps, but doesn’t complete the full story. A better (and more interesting) approach is to learn scales that connect with each chord of the 12-bar progression.
For each of the blues chords, there is a scale of pitches that fits. This scale is known as the the mixolydian scale.
THE DOMINANT SCALE (MIXOLYDIAN)
The Mixolydian mode, is the fifth of the seven musical modes. It is similar to the major scale except for having a lowered seventh. The Mixolydian scale is the scale that is generated when a major scale is played with the fifth note (fifth scale-degree) as the root. Thus, a C major scale played from "G" is a G Mixolydian scale. This is why the term "mode" is more appropriate than "scale".
The G Mixolydian mode is the same as a C major. So what's the difference? There is no difference; it's the chords that create the magic. Playing a G Mixolydian scale over a C major chord will sound exactly like playing a C major scale (because they are identical). However, playing a G Mixolydian scale over a G major chord will sound "Mixolydian."
Learn the following "Mixolydian" modes that can cover each of the chords from our 12-bar blues in example one.
"C MIXOLYDIAN MODE"
"F MIXOLYDIAN MODE"
"G MIXOLYDIAN MODE"
When analyzing these scales what we notice right away is that each Mixolydian mode's natural blues structure tends to give us a bit more tolerance for notes that may not fit perfectly. If we played some of these sounds outside of the blues, they could sound off. But in the blues they sound great.
Most improvisers will use either a C major pentatonic (C, D, E, G, A, C) or a C minor pentatonic (C, D, E♭, G, A, B♭) over the C blues.
However, if they use the C minor pentatonic, they will often bend the E♭ up to E, especially over C7 tonic chord. And in addition they will sometimes encounter licks that use the other pitches we’ve talked about. But in reality, you can use both C major and C minor pentatonic scales over the same blues.
Pentatonic Major + Pentatonic Minor
(The Blues Combined Scale):
When dealing with a blues situation, you can generally draw on the Combined Blues Scale as a primary source of pitches. Then you can add color notes to the blues, which are generally context dependent. Color notes are any notes that we discussed already, but which are not in the Combined Blues Scale.
Repeating the ♭3 to ♮3 effect in many cases, opens the door for other “twists” and re-uses of the same idea in another contexts of the 12-bar progression. The ♭3 to ♮3 of the blues (E♭ to E in our example) is one case of this. Because the Combined Blues Scale is very heavily defined by the sound of a ♭3 of the I7 resolving up to the ♮3 of the I7, this relationship can also be used on the other chords.
In the case of the C blues, the F7 has “A” as its ♮3. This note is sometimes preceded by A♭, emulating the relationship first established on the tonic chord. That introduces a new note, A♭, into the blues. The same can happen for the V7 chord, in this case between B♭ and B, but both notes were already in the blues, so no new pitch is added, and it’s a little less special.
The “Blue Note”
We still haven’t talked about the famous “blue note”. This term is most often used to describe another color note that has similar properties as we've noticed from the scales discussed above. The blue note is between 4th and 5th of the tonic chord. (G♭ in our example below).
The "C Blues Scale"
The blue note (circled above) is almost always approached from below the 5th. (G in our example). The application of the blue note is almost always done as a very short duration, not long enough to sound dissonant, (often performed as a grace note).
The blue note almost always resolves up to the 5th of the key, or back down to the 4th. However, using the term “blue note” to refer only to this note is a bit of a misnomer. Actually, many notes that we’ve talked about can be thought of as blue notes.