Watch the Lockwood online guitar lesson by Matthieu Brandt from Jump Blues
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 / bass lines. This way of accompanying is pretty thick, so if you're using it, play behind the singer or soloist, meaning soft.
The major third of the chord is kept constant, while you play another line on top of it. These examples can be challenging, because of the right hand picking movement.