7th? and blues

The seventh chord used in blues is the dominant seventh, with a flatted seventh. That note's not found in the underlying major scale, but then neither are several notes in the minor blues scale we play over those chords. The dissonant tension of the seventh chord just sounds good.

Exactly. And if you don't like how it sounds, play another chord. Theoretic understanding won't explain musical taste nor is it supposed to do that. In general dissonant notes are universally considered to be cool in blues. Why? Who knows...

The 'blues scale' is 1-b3-4-b5-5-b7, so the b7 of a dominant 7th chord is present in the scale.

It also means there's always conflict between the natural 3rd in a chord, and the b3 in the scale. That sets up the tension that Ricochet mentioned.

When you use 7ths for all chords (I7-IV7-V7) in a progression, it's normal to use just a major chord (or a 6/9 chord) for the very end, though - it kind of resolves the whole ball of wax.

BY dropping notes down in the scale alter the feeling of the scale gives it musical color from with some call Blue Notes and can make you tone darker or sad sounding. The third is the middle note in the I chord. By dropping this note ½ of a step, which makes the A minor chord is sad sounding. The flat third of the V is also the flat VII when you count up from the I chord. The flat VII is a very,important for color as a transition note. Learn the Diminished 7 chords, these are the Blues chords.

The flat 7 should be the starting point or ending point of many of your riffs. Here's some great informationn on the subject.

Blue Tone and Passing Notes

Joe

Not to confuse things, but a diminished 7th is a flatted dominant 7th, which makes it a double flatted major scale 7th (same note as the major scale 6th).