Blue notes are a hallmark of blues music. What is a blue note? ...

[luminitza]

Blue notes are a hallmark of blues music. What is a blue note?
 
  What will be an ideal response?

Question 2

Explain what a blues scale is. Show a blues scale on G.
 
  What will be an ideal response?

[CAPTAINAMERICA]

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 3^rd, 5^th, and 7^th scale degrees (shown as underlined) along with the other usual scale degrees of a major scale (the 7^th scale degree is F in the key of G major).