What about the natural 3rd? Why isn't it part of the scale? It seems
that there are good reasons to include it in the scale.
This very question was posed by Bill Monroe and his fiddle player Kenny
Baker. They felt that the flatted third had burned its way so completely
into the ear of their audience that the natural third, coming where you'd
expect a flatted third, was actually more compelling as a blue note. I am
not sure when they started substituting the natural third in for the
flatted, but it would probably be in the late 50's or early 60's.
This was told to me in the early 70's by a guy named Bob Fowler, who had
just been Monroe's guitarist for a few years, while Baker was still in the
band.
As with other highly formal Japanese arts, sumi-e ink painting places
various kinds of balance requirements on the painter. If you paint an
object on one part of the field you are required to place one in a
geometrically opposite location, to balance it. My favorite solution to
this problem was done on a three-screen work: A bird is darting just off
the 'ground', all the way at the right edge of the right screen, flying to
the right. As we often see when a bird is darting low to the ground this
bird appears to be moving very quickly. In the blink of an eye the bird
will be gone, except of course that this is a painting, so it's there
forever.
But the sense of the bird's high speed is so convincingly executed that
your eye actually seems to see the ghost of a black line all the way across
the bottom of the painting - the blur of the bird's flight path. There is
no black line, but the feeling that there is one is enough to satisfy the
formal requirement for balance.
Monroe and Baker's blue-note innovation is parallel to this, for me
anyway. When they play the natural third as the blue note it sounds even
bluer than the flatted, because the brain somehow hears the echo of the
flatted third, too. Very, very powerful.
Musical theory is essentially the recipes of how composers and players
achieved their effects IN THE PAST. Every musician eventually adds to the
Theory when they achieve effects outside the existing theory. Zvi's
question is the kind of question one asks when one wants to make music that
is one's own.