Watch the Common Country Blues Forms online guitar lesson by John Hatcher from Fingerstyle Blues Roots
By far the most common blues form is 12 bar blues. You can hear the 12 bar blues form at work in songs like "Sweet Home Chicago" by Robert Johnson.
Think of 12 bar blues as a framework that can certainly be altered and tweaked for style and expression, but the familiar groove and cadence of a 12 bar blues serves as a great launching pad for much of what we do on the guitar.
Another very popular song form in the blues is 8 bar blues. There tends to be a good bit of variety in the 8 bar pattern, but you've got to know a few standards like "Key to the Highway" and "Worried Life Blues".
The third common song form that we'll look at comes from ragtime music. Back in the early part of the 20th century, ragtime music, short for "ragged time", was gaining popularity and the blues was just putting it's boots on. This led to some crossover, and you'll hear ragtime influences throughout the blues, most notably in the style of players like Blind Blake and Blind Boy Fuller. Even Robert Johnson used this ragtime sound in his outlier tune, "They're Red Hot".