Watch the Basic Blues Shuffle online guitar lesson by TrueFire from Play Blues Guitar 1: Quick-Start for Beginners
The open string blues shuffle might be the most iconic "blues sound" there is. Learning this one will instantly give you a "bluesman" button to wear!
Take your time with this and make sure you have the timing down. You don't want to brush over this even if it seems simple, it is the foundation for everything you will learn in the blues.