Answer to Question 1
A blue note occurs when both regular and flatted forms of certain scale degrees are used (often with sliding in between these scale pitches)see also question 2 above regarding intonation and blue notes.
Answer to Question 2
A special scale which includes the flatted (lowered) third, fifth, and seventh scale degrees in addition to the usual (un-flatted) scale degrees of a major scale. For example, if beginning on the pitch G the blues scale, including the flatted scale degrees is:
G A Bb B C Db D E F F G
Notice how the blues scale on G expands the usual G major scale to include the flatted 3rd, 5th, and 7th scale degrees (shown as underlined) along with the other usual scale degrees of a major scale (the 7th scale degree is F in the key of G major).
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