Well Jimolly, western music divides an octave into twelve equal parts, called 'semitones' - this is the distance between two adjacent notes on a piano, or moving up one fret on a guitar. We are used to hearing music that uses this 'chromatic scale', but in many traditional cultures, they break up an octave in a different way - the notes they use are sometimes in between our 'semitones'. We refer to these notes as 'microtonal'. Africans are fond of a note which is somewhere between a minor and major third in reference to our chromatic scale. When african slaves were taken to America, they found a way to capture this sound on western instruments - by playing a major type chord (featuring a major third), and singing a minor third over the top. The dissonance created thus gave an impression of the microtonal note they were seeking. This is the quintessential 'blue note' - the sound of the blues. These days, we refer to a 'blue note' more loosely, as the scale degrees flat 3, flat 5 and flat 7. Their 'blueness' is heard most dramatically when used over a dominant 7 type chord. I hope that this has been educational and not too long!