If your playing in the key of C for example you would play your blues solos in A minor pentatonic, Blues pentatonic. A minor is the 6th chord in the key of C and works well over all the chords in that key. You could also play the Mixolydian mode which in the key of C would be G7. The Mixolydian mode is the same as a major scale the only difference is the 7th is flat so it would be a F instead of F#. Don't get me wrong these are not the only scales/modes you can use in blues but they are a great starting point. Make note that just because your playing the most common blues scales/modes doesn't mean your playing the blues. It's all about the feel,mood and choice of the notes you play.My suggestions would be to learn these scales mentioned and listen to as much blues lead playing as possible. Mocking your favorite guitarist is a good way to learn if not the best way. Hope this helps

Thanks Brian Calloway