The folks over at Texas Blues Alley recently (ish) posted a lesson video on the subject of how and when to use open strings in a blues context.
The video takes you through a few keys, and shows you how you can use open strings in each.
Though there are no accompanying tabs with the lesson, the camera angle makes it fairly easy to see exactly what's going on. You can watch the lesson above.
Be sure to check out Texas Blues Alley’s YouTube page for more great lessons.