Hello Sammy and welcome to the forum.
Not all "Blues" will follow the basic 12-bar form.
I look at it like this:
ZZ Top's "La Grange" is an example of a song that pretty much repeats the same figure on the I chord for the entire song.
The "figure" is actually just a riff using the 1-b3-4 of the minor pentatonic scale played as double-stops/power chords.
(It's actually a variation of what Griff calls the "Reverse John Lee Hooker style variation in A" in his 52 Rhythm Fills And Variations course, Ex 1-12. A variation on a variation!!
)
They play the same figure (Ex 9-1) on the I chord, A, for most of the song and then move it up to a different key, C (Fig 9-2), for the solo. It's the same figure, just moved up 3 frets. C becomes a new I chord. Then they bring it back down to A after the solo.
If you did play this figure through a I, IV, V 12-bar progression in A it would go something like this:
A5 C5 D5 for the I
D5 F5 G5 for the IV
E5 G5 A5 for the V
This is what the MG's do with the "Green Onions" riff over the I, IV, V changes in F, using yet another different variation on the rhythm.