Watch the Single Note Blues Riff online guitar lesson by TrueFire from Play Blues Guitar 1: Quick-Start for Beginners
Not all songs are built from chords. Some are built from riffs, or basically single note patterns. So the interesting thing is you have to make sure you are playing the notes cleanly and not being sloppy with it in order for it to sound right. Keep your picking efficient and don't let your picking hand get out of control! Blues rhythm guitar is all about getting the changes smooth!