@Jerome: It was more like 1/2 way between flatted and natural. The quote in this wiki article is a decent summary. http://en.wikipedia.org/wiki/Blue_note
Rachmaninoff was a frickin' stud.
Quick tangent on non-western theory:
I studied folk music of Lao river people for a college music thesis. During the course of study I learned that their scale was very very close to our major scale, with a few differences. They don't think in terms of scales and their instruments are always tuned by ear. So the instrument (called a khaen or caen) has a scale that is close to our major scale in terms of intervals but there was no common tuning, i.e. a440.
Because the instruments are hand tuned by their maker and made from pretty cheap materials (long pieces of river grass and old melted down Thai coins), the intervals were tuned inconsistency relative to how we hear music...so some notes would be off by a few cents in either direction.
At any rate, the tuning was close enough that I was able to transcribe a portion of the Lomax Anthology of American Folk Music for playing on this instrument, using a system of notation that I developed for the instrument.
What is really fascinating is that these river people developed this music and this modality with no influence from the west.
