Due to its makeup, the dominant seventh chord is rather unstable. Especially its major third and its minor seventh with the tritone interval between the two (the famous devil’s interval supposedly forbidden in the music of the Middle Ages). It tends to resolve in a movement of V-1 (perfect beat) on a major or minor chord. But this has changed with the blues in which this chord acts as the first degree.
On the blues major scale, it is therefore very common to find dominant seventh chords on all degrees. And in the styles derived from the blues (Jazz, Rhythm'n'Blues, Funk, Rock) we will also find dominant chords on different degrees.
Let's start with the blues grid. It is built around three chords placed on the first, fourth and fifth degrees. In a minor blues, these three chords are major, sixth, and a dominant seventh (with possible extensions of ninth and thirteenth). In any case, even if it is not played, the seventh minor is implied. Here is an example of a blues grid of twelve measures, one of the most common forms of blues, in C (C7 F7 C7 F7 C7 G7 F7 C7 G7).
In the Blues songs "Swedish Blues" and "Blues for Alice" by Charlie Parker, the first chord is a Major Seventh chord, which does not prevent the playing the seventh minor during improvisation. In Jazz, it is common to replace a number of major chords with dominant seventh chords.
For the minor blues, the chord of the first degree will be minor to the minor 7, and sometimes the minor 6. There will also be a seventh dominant chord on the fifth degree, sometimes on the fourth, and the chord built on the sixth degree.