Form
Every chord is a 7th
I7, IV7, V7 — dominant sevenths, all three. Even the “home” chord carries a dominant's tension. That's the blues sound.
The quick change
Bar 2 swaps to IV a bar early. You'll hear this far more often than four straight bars of I.
The turnaround
Bar 12 lands on V instead of I, pointing back to the top so the form can repeat without a hard stop.
Shown in the key of A. It's still I–IV–V in any key — spell those three chords from wherever you start and the form is the same.