Dominant 7 chords are the blues chords. In blues you will often see just a series of dominant chords. G7 C7 D7..etc. Sometimes they are altered as well, a common dominant alteration is the dominant 7th sharp 9 chord (G7#9). The blues sound is generated by harmonic minor 2nds. WHich is playing two tones a half step away. In a chord, this is the same, the b3 over the natural 3rd (G7#9). The b5 over a natural 5th (G7b5). This doesn't occur with the root as much though. The dominant seven flat nine chord is not as common as the others in blues. If you use them, be careful and make sure it's the texture you want.