Watch the Dominant Soloing 2: 18 online guitar lesson by Brad Carlton from Bluesology 1

The diminished 7th chord, a restless chord demanding some kind of resolution, is going to move our blues progression along with more tension. After discussing the construction of the diminished 7th chord(1-flat3-flat5-double flat7) we will use it as a passing chord moving from the IV to the I chord.
You will hear the #iv dim chord in measures 2 and 6.