The chords in bars one and five are the resting chords; all
of the other chords could be considered active; thus, you play the appropriate
scales for the duration of the resting chords, and the blues scale of the OVERALL
KEY for the duration of the active chords.
In this particular case, the resting chords are both Major
chords, but Minor chords can also be resting. Look for Major or Minor chords
that are preceeded by their Dominant V chord (or a Dominant chord a halfstep
above the root; if it's a Minor chord, then check to see if that Minor chord
is followed by a Dominant chord a perfect 4th above it's root. If so, it's an
active chord (a candidate for the blues scale approach); otherwise, it's probably
a resting Minor chord (play the appropriate Minor scale).