Watch the Alt Thumb I VI II V online guitar lesson by David Hamburger from Fingerstyle Blues Factory
The title says it all. In the second measure, we use a bit of rationalizing to keep the alternating bass going beneath the descending blues scale melody. Basically, by calling the bass note on beat 2 the augmented 5th of F#7 (and therefor an o.k. “altered tone” to play on that chord) we can get away with using an open string in the bass at that point. Likewise, using the same open note on beat 4 is a bit of cheat too; it’s the #9 of B7, which is not really an ok bass note but since it resolves quickly to E, anticipating the I chord in the next measure, our ears hear it as just a suitably bluesy moment. All of which frees us up a bit to put more of our attention (and fingering) into the lick happening on top.