From a western classical perspective, the 12-bar Blues progression betrays a strong plagal character and to our classically-trained ears, the V-IV move is a very striking moment. Can you figure out the plagal expansion in this basic version of the progression:
I I I I
IV IV I I
V IV I I
In practice, Blues musicians typically add a minor seventh to each of the chords. Or, for more variety of harmonic colours, they may add a 6th instead or play with substitute and/or altered chords. Oftentimes too, turnarounds are added. A very common simple example is the replacement of the final I with V7.
Listen to this online example; note that
(i) the added-sixth is labelled as I6, which in our case means tonic chord first inversion; hence for us, we may label it as I+6 instead. We may also label it as iv6/5 and take it to be a tonic substitute chord.
(ii) the I7 in the online example would be Ib7 in our case; it is not a V7/bVII. Functionally, it is an enriched tonic function chord.
(iii) some of the chords can be inverted to effect a different bassline (not illustrated in the audio clip)
Other variants of the Blues progression are given on the above online music dictionary:
12-bar minor blues - note the reversal of the typical V-IV progression, here involving a substitute chord (ii7-V7 instead of IV7-V7) with added 9th and altered fifth.
8-bar blues - note the use of Bdim7 as tonic substitute and the ii7-V7 progression; the VI7 may be taken to be a turnaround chord, propelling the harmony towards ii7 (can you understand why?)
Jerry Lewis's Boogie-Woogie riff - see how the neighbouring six-four is incorporated