Another interesting sound used to create Blues riffs is the Mixolydian scale, or mode. Its degrees are 1, 2, 3, 4, 5, 6, and b7. Here it is in the key of A.
Here is a 12 Bar Blues riff based on the mixolydian scale. You have probably heard this one many times.
Here is another riff derived from the Mixolydian mode. Observe the fingering carefully and pay special attention to the rhythm.