Using the Caged Guitarist® system you can see why any notes can be played over the blues using dominant seventh chords.

NB: The Dominant seventh can handle any note as either an extension "9, 11, 13", or as an alteration "b9, 9#, b5 (11#, 5# (b13).

Together with chord tones R, 3, 5, b7, this leaves just one more note (7) out of complete chromatisism, ie: every note. And as long as you pass over the natural seven (7), it will not sound wrong.

Therefore you can literally play any note over a blues.

This is much more clearly understood and visualised using Caged Guitarist® interval shapes. In this case the C/shape, E/shape and G/shapes for F7, Bb and C7.