One of the pioneers of Chicago blues accompaniment is Robert Junior Lockwood, who recorded with Sonny Boy Williamson, Little Walter, Muddy Waters and a bunch of other Chicago greats.
As you can see in the next examples, the guy deserves a statue himself.

These examples use intervals combined with melody / basslines.
This way of accompanying is pretty thick, so if you’re using it, play behind the singer or soloist, meaning soft.

The riff that R.J. Lockwood built his career on uses notes from the mixolydian scale and harmonizes them (2 or more notes played at the same time).

The flatted third is used as a guiding tone to the major third.
Keep your middle finger on the sixth fret (the major third of the scale).

If you’re playing through a blues progression, just follow the chords and move the pattern up the neck to the IV and V chord (Example 2).

Keep a good eye on where the root of the chord is and you’ll know where to start the riff.
Use the root on the D string as a marker.

The riff on the IV and V chord can also be played at the same position as the I chord riff (Example 4).
Just move this riff up the neck two frets and you’ll have the V chord riff.

(Examples 1 and 3 can be found in the Swing- and Jump Blues Guitar e-book)