This super heavy blues riff is in the style of the band Cactus and their tune "Evil." Cactus was initially conceived in late 1969 by former Vanilla Fudge bassist Tim Bogert and drummer Carmine Appice, after plans to team up with guitarist Jeff Beck were sidelined when Beck had an automobile accident that took him out of the music scene for over a year. They brought in guitarist Jim McCarty and singer Rusty Day, helping round out the band's heavy sound that lead people to refer to them as the "American Led Zeppelin." Although only active for a short period of time, the band has had a lasting influence on blues rockers everywhere.
The form here is a basic 12-bar blues in E, and as we see quite a bit in blues rock the third of the chords is left openly ambiguous. This definitely leads to a heavier bluesy rock sound.