Forget modes, chord tones is really straight forward.
Each chord is made up of some notes. In blues you generally use the what is called the dominant 7 chord.
So a 12 bar blues in A would be A7, D7, E7 (I, IV, V or 1, 4, 5).
The A7 is made up of A - C# - E - G (1 - 3 - 5 - b7) or 5th fret of the E string, 4th fret of the A string, 7th fret of the A string, 5th fret of the D string.
I'm sure that is very familiar if you jam the blues already.
If you take the last note off that and just use A - C# - E that will be an A chord. So you should now by removing a note from what you already know, be able to jam on a few more chords.