Blues notes are alterations of the chord that are a "carry over" from guitar players. Years ago when the blues style was first being formed, guitar players would 'bend' notes on the guitar. This created a very soulful technique because the pitch would move up or down a half step or more.
However, it is nearly impossible to bend notes, unless you actually manipulate the strings inside the piano! So what is the pianist to do? Blues notes to the rescue!
By creating an alteration on the 3rd and 5th chord tones, we can simulate that blues, soulful sound. Example A shows you a regular C7 chord and chord tones.
Now, if we add alterations on the 3rd and 5th chord tones, we get example B. Notice that the minor 3rd resolves to the Major 3rd and the #4 resolves to the 5th.
There are basically three ways to use these blues notes in your playing.
Listen to Example C:
Listen to Example D:
Listen to Example E: