James Alley Blues
The intro in this version is one full iteration of the 12 bar form.
The first four bars of D are replaced with a riff constructed from a hybrid of the D pentatonic scales (DEF#ABD') and the D blues scale (DFGACD'). The notes in the riff are primarily from the pentatonic scale with the exception of the F from the blues scale. This is a common technique in the blues. The F is the b3 of D major and one of the primary "blue" notes.
The A and D chords both rely heavily on the alternation between the respective chord's 5th and 6th (E-F# for the A chord; A-B for the D chord). The alternation between 5th and 6th also plays prominently in the riff on the second bar of the G chord (D-E).
A variation of the opening riff is reprised in the final two bars of the form, acting in place of a harmonic turnaround.
I recommend listening to the track without the accompanying "groove." This is originally for solo guitar and voice.