This is a very legitimate question and one that torments more than one improviser in search of a logical explanation?
Like you, I have often asked myself this question even though I was already capable of analyzing much more complicated chord charts. But it must be admitted that the blues holds a special place with its logic which defies the elementary rules of harmony. Without claiming to have the truth, I nevertheless have several answers to offer you.
Major blues (yes, I like to write this word with a capital M) is the result of the meeting between the music of Europe and Africa and it does not respond to the rigidity of so-called classical harmony. This is why it is difficult to want to give rigid and categorical answers to explain the harmony of this music which, in essence, aspires to more flexibility and freedom than most Western music.
The blue notes
And yes, I say THE “blue notes”, because it’s not just the one that’s stuck between the 4th and 5th!
The blue notes would in fact be inflections of the 3rd, 5th and 7th of a Major scale. This is why there are these notes present in the constitution of what we call the blues scale.
But in fact, there is much more than these 6 notes to the blues. (The minor pentatonic plus the blue note) With the inflections which oscillate between the blue notes and the notes of the Major scale, this
makes about ten different notes possible! And they are used, naturally by singers, but also by harmonica players who twist certain notes downward using the “over blow” technique and other alterations, and also guitarists with “bends” techniques. The blues scale is therefore a scale with the 3 “blue notes” which highlight (or rather in sounds) the particular harmonies that we hear in the blues.
The use of seventh chords
7th chords have a very specific function in so-called classical tonal music. By the presence of a particular interval of 3 tones (called tritone) between the 3rd Major and the 7th minor (or dominant), this chord in harmonic tension aspires to resolve towards a more stable tonal chord. This is the case of perfect cadence V I.
In the blues, we use the 3 major degrees of the major system, the I, the IV and the V, but these chords are often embellished with a minor 7th, which is inaccurate within a single tonality from the point of view of classical harmony. In a Major tonality, only degree V has a minor 7th, and degrees I and IV have a Major 7th . But aesthetically, it holds up. Besides, if you are reading this article, it is because you are sensitive to this music! So now, take a good look at what happens with the use of these chords:
The 7th of these 3 chords are present in the blues scale and in particular, the 7th of degree IV which is the minor 3rd of the tonality and therefore of the blues scale. Which explains the use of the minor 3rd on a Major blues, in any case, at least when playing on the degree IV7 chord.
The Major tonality is therefore distorted by the use of these 7 chords. Clearly, we could play a Major melody on the chords of degree I7 which naturally includes a Major 3ce, then, arriving at the chord of degree IV, we could replay this same melody with the same notes except for the 3rd of this melody which would become minor. This melody is therefore still anchored in the starting key, but it is influenced by the change in harmony that accompanies it.
In jazz, there are a large number of blues or blues-infused themes that meet this characteristic:
Now's the time, Tenor Madness, Sweet Georgia Brown, …
The 7#9 chord in the blues
The 7#9 chord is a chord that includes a Major 3rd, a just 5th (in theory but rarely played), a minor 7th and the #9. This last note is therefore the equivalent of the 2de augmented above the octave.
To the ear, this note sounds like a minor 3c even though there is the presence of a Major 3c in this particular chord. It is often used in jazz as an altered chord, that is to say a chord placed on a degree V which seeks to resolve towards its degree I. But in blues, it is very different. On degree I, it is the emblematic chord that brings together the 3rd Major and its “blue note”. It sounds blues all by itself! When I think of this pairing, I think of Jimi Hendrix in “Purple Haze” or “Foxy Lady”! By playing on this chord, we can therefore play with a Major scale (Major pentatonic or even the mixolydian mode) as well as with the blues scale and its minor 3rd which will go very well with this #9.
Even if “#9” is not played on a degree I, the improviser will imply this “7#9” chord color by playing with a blues scale or a minor pentatonic on this Major chord. And as there is not this note in the chord of degree V (the third chord played in blues), we can therefore improvise on the whole of the Major blues with a pentatonic or minor blues scale.
If you want to know more, I show you all that and more in this training:
When you have a music theory score, (if it is correctly written), the sharps or flats present just after the treble clef give it to you. We call this key armor.
Example: Have you met Miss Jones:
