Watch the Chromatic online guitar lesson by David Hamburger from Fingerstyle Blues Factory
You can think of this lick as a kind of variation on the Hubert theme. In this case, we open with the full fingering of the B7 voicing, playing the second fret on the high string with the pinkie. The first chromatic move comes as an anticipation of the A7 chord at the end of measure one with the half-step move from C to C# (the minor and major third of A). Next there’s the move from B to Bb to A coming into the third beat of measure two (which you can look at as the 5, b5 and 4 of the E blues scale) and finally, there’s the move from G to G# at the end of measure two, resolving from from the b3 to 3 of E.