Twelve bar blues only indicates that there are 12 beats to a bar in a very simple chord progression of three chords. It is called a I, IV, V chord progression. For example, Johhny B. Goode is your basic 12 bar blues in the key of A. Crossorads by Cream, however, is the same three chords(A, D, and E) but is considered a 16 bar blues. Listen for the changes...they don't follow the same pattern. As to who invented it...good luck but it seems to me like it is basic music theory.