I can try to help you out, but I don't play much blues in the John Mayer type-style. But it seems like a lot of the John Mayer stuff is just kind of doing little blues licks with double stops and stuff. What you can do is take the scale belongs to the chord and kind of fiddle with it using two notes at a time. Like
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e--------------------
b---8----------------
g---9--8h9-----------
d---8--8---10-8-------
A----------------10---
E---8---------------8--
sort of uses the C blues scale over a C7 chord. The Eb hammering on to E natural will give it that "blues" type sound because of the Eb (b3 of the chord) over the Dominant 7th chord (which has a major third, not a minor). When you hammer on to the E natural and hold the the Bb on the D string, you're outlining the chord because those are the third and 7th of the chord.
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e-----------------------------11p8-----------------
b---10--10--------------------8-------------------
g---8h9-8---------------------9-----/9-12--8/9----
d---8---8-----10--------------8-----/8---------8---
A--------------8h9---------------------------------
E-------------------11/12--8----------------------
There just sort of using the mixolydian mode and the blues scale. Basically if you dick around with taking a voicing, (since we're talking about this blues thing, generally dominant 7th chords) and playing portions of the chord while improvising using scale tones (from Mixolydian mode or blues scale).
Sorry if this is unclear. Let me know if this makes any sense and sorry about the shitty tabs.