We begin with a series of double stops that works equally well over the I and IV chords, because the notes avoid the major 3rd of Bb (7th fret G string). The major 3rd of the I chord is something you pretty much want to avoid when you're playing over the IV chord.
In the 5th & 6th measures, take note of the ascending/descending move that transitions into the turnaround—it's basically the same move that we've been covering (over and over in various forms) but in the 7th & 8th measure of a 12 bar blues progression. It's also a bit different here because we're using two note chord fragments on the G & B strings. Since these are very small two note chord fragments, often these notes will cover more than one chord. For instance, when we grab the 10th fret of the G & B strings (right before the II - V - I turnaround), those notes could spell out either a G9 (6th of Bb) or Dm (3rd of Bb). The more notes you add to a chord, the more defined the chord becomes.