The blues started off as a folk form. In its simplest form, a statement is made. It is repeated. It is then answered.

"got the blues, blues down to my shoes...
got the blues, blues down to my shoes...
gimme a listen, here's my latest news..."

This folk form was often sung with accompaniments of various length...13, 11, 14.5 or any other bar length would be used to fit the lyric and the intent.

Cut to the early 20th century. In New Orleans, European-trained musicians, having difficulty with what they saw as wild undisciplined structures began to squeeze the blues into stricter 12-bar structures, four measures for each phrase. A new, hybrid-style blues was forming.

This is the blues progression at what we think of as its most basic.