Consider the standard boogie-woogie bass such as this one. The C, F and G phrases are each in its own mixolydian. Is that a modulation from C to F and G? If no, why?
No. The F and G are heard as the subdominant and dominant of the home key. When listening you have the sense of being away from the C chord and wanting to return to it. A modulation sets up a new home key. It can be temporary (like a vacation home) or permanent (like moving to a new home, for instance the modulation just before the last chorus of "Grandma Got Run Over By A Reindeer").
I suppose you could consider it a modulation; but it's normally considered part of the "changes".
It's as if C-mixolydian, F-mixolydian and G-mixolydian were all superposed together to create a C-"blues" scale. C-D-Eb-E-F-G-A-Bb-B-C. The notes that belong to the "superscale" but not the "current" scale are the blue notes.
But the "full" blues scale is constructed by using the dorian scales: C-dorian, F-dorian and G-dorian, giving: C-D-Eb-E-F-F#-G-Ab-A-Bb-B-C. This gives you more blue notes against the mixolydian harmony.