The Georgia Sea Island Singers link was from me.

A couple thoughts:

1) More likely that anything else, "The Blues" were called "The Blues" because so many of the songs made reference to having the blues--

Went to the river
The river was runnin' up and down
Went to the river
Had the blues so bad
Started to jump in and drown

2) It is true that many blues tunes use the 12 measure phrase--but a lot of blues, particularly early and folk blues, doesn't..

3) Though a lot of blues use a fairly standard I-IV-V chord changes, there are lot of others, and, in fact, there are blues that don't have any chord changes.