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The Blues scale: Blue Notes

The note which in classical music styles determines if it’s a major or a minor scale is the diminished (flat) third. In Blues music, it usually does not reach the exact target note; it is often a bend form the minor note into the major note.

The note which is part of the dominant seventh chord, the one which leads back to the tonic note is the diminished (flat) seventh.

Another definition you’ll find is that a Blue note is at all times played at a lower pitch than those of the major scale to express a certain feeling. Non-equal tempered scales are used by the African roots of the Blues music. Classical notation is used to describe these in-between notes. You can consider American Blues music as a well grown combination of African and European music styles.