Watch the Blues Study in D 4 online guitar lesson by TrueFire from Acoustic Guitar Greatest Hits Vol. 1
Lesson Source: Tim Sparks's Post-Modern Fingerstyle Blues
The bass line's feel is created by playing 6/8 against 4/4; that is, you create the familiar pulse of a 6/8 even though the rhythm is counted in steady 4's. Getting the feel right is the central to playing the part, so get familiar with the bass exercise shown, including all the hammers and pulls, before trying to marry the bass to the melody.