First of all it should be mentioned, that a blues is an ordinary song of three chords, the major blues is a major song, the minor blues is a minor song. The overwhelming majority of blueses are written in one key, and almost all of them have a twelve bar formula, arranged in the same manner.
What is the blues phenomenon? It is more than a hundred years since the blues appeared on the world music stage, and the interest in it is growing. The point is in so-called "blue notes" or more precisely in the color they added to the sound of a major key.
Different sources give us different variants of a blues scale, which can be summarized this way. The simplest blues scale looks like the minor pentatonic with the added triton. If we analyze its structure, we will see that it consists of the main mode degrees: I (the first), IV (the fourth), V (the fifth) and of blue notes - the lowered III (third), the lowered V (fifth) and the lowered VII (seventh) degrees, they are also called minor. At the same time this is the major key, so, naturally, here the Major scale works. The superposition of these two scales represents a complete blues scale. (Look at the picture below)
The minor blues is not as harmonically rich as the major one. The simplest blues scale, as it was mentioned above, almost coincides with the minor pentatonic, except the lowered V (fifth) degree. We have already discussed in previous parts, that parallel keys differ by three accidentals, but the tonic and the fifth are common. According to the definition, parallel keys are not even closely related. This great difference between parallel keys is the reason for its blues sound. If we play the Major scale over the major blues, and lower its III (third) and VII (seventh) degrees, then our scale practically turns to the Dorian minor. This way a sound of one key superimposes with a sound of another key and brings our ear into a state of a polytonality. So it is better to have a good ear for music to play the blues.
The blues scale can also be examined in five diatonic positions. Pay attention that five blues positions are very like five pentatonic positions.
Below you can see some number of blues sequences. Every line of this table represents a twelve bar blues. Double vertical lines divide every blues into three logical parts and are often marked by letters "A, A, B" or "a question, a question, an answer". Harmonic sequences are noted by Roman numerals (meaning mode degrees). This notation is universal and does for every key. You should only convert numerals to notes of a chosen key. Analyze this table and pay attention to the similarity of A, A, B parts in different blues sequences. First four bars almost always begin with the I (first) mode degree; next four bars from the fifth to the eighth inclusive almost always begin with the IV (fourth degree), and the last four bars from the ninth to the twelfth inclusive also almost always begin with the II (second) mode degree.