Could I play relative minor blues scale or dorian to a piece? Let’s say
that my blues song or jazz piece is on C7...could I use the A blues or A
dorian scale since A is relative minor to C?
The mode that is relative to C Mixolydian (one of the proper scale matches
for C7) is A Phrygian, which is basically the same thing from a different
note.
A dorian has a B natural in it which will clash with the Bb of the C7 chord.
The Blues Scale would work because it jumps over any problem areas in
the relationship between the scale and the chord.
How can I learn what scales to play over each chord of the song as the
progression moves along?
My best advice here is to find a good teacher who can guide you through
all the complex choices available to us as players.
This is the topic of much of the theoretical study of jazz music. There is a
lot of information on the internet about this, but not all of it is correct and
it can be confusing and overwhelming if you don't have a plan!
How can I easily understand chord/scale relationships?
Examination of the Harmonized Scale can elucidate the choices available
to you. Scales that are used with chords must contain all four Chord Tones
and have the proper color notes to help the chord move as it should with
the proper energy.
Placement within the key/scale is also important to the chords function.