I would say that the basic idea of the (12 bar) blues is this:
4 bars with the feel of I
:
Could be as simple as I | I | I | I
, or as complex as I | vii III7 | vi | II7 | v I7
(notice that in the important places, the harmonic onbeats, the chords are I
, and vi
(the relative minor), giving the feel of I even though that chord doesn't appear much.
4 bars, 2 with the feel of IV
and 2 with the feel of I
:
Traditionally this is IV | IV | I | I
, but it could be something like IV7 | iv bVII7 | iii VI7 | biii bVI7
(even though there isn't much sense of I
in the the latter 2 bars, it still sounds like a blues due to the context).
4 bars, 2 with the feel of "coming back to I
", and 2 with the feel of I
:
This could be V | IV | I | I
, or along the lines of | ii | V7 | I VI7 | ii V |
(a ii V
heading back to I
, and then a turnaround in I
.
The idea of the blues can also be extended to different numbers of bars.
For instance, in Grease, virtually every song is a blues (or I IV vi V7
(Those Magic Changes)), but only one is 12 bars long.
As I remember, there is also a 14 bar blues:
I | I | I | I
IV | IV | I | I
V | IV | V | IV
I | I
a 10 bar blues:
I | I | I | I
IV | IV | I | I
V IV | I
and an 8 bar blues.
The 16 bar blues also exists:
I | I | I | I
IV | IV | I | I
V | IV | V | IV
V | IV | I | I
The chords in the blues can also have their quality altered - a traditional blues is likely to have all dominant 7th chords, a more modern blues might have higher extensions, and you can also have blues in minor keys.
The blues are also linked by the use of the blues scale to improvise over them (1 b3 4 b5 5 b7
) although this scale can also be used in other situations (and often seems to be overused).
N.B. The complicated blues I gave as an example is the Bird changes (that page also lists a couple of other blues for comparison).