Tritone substitutions are one example.

Think about a 12 note scale. The note that divides that in half is called the tritone. So, if you start at B, the tritone is F (E's tritone is Bb, C's tritone is Gb, etc.).

Now when we have dom7 chords (as most chords in a blues are), we can keep the same quality, but substitute the root for the tritone. When we do this, we add the #11 tp the tritone, which ends up giving us the root that would have been.

The chord tones of B7 are B, D# (Eb), F#, and A.

The chord tones of F7#11 are F, A, C, Eb, with B as the #11.

If you'll notice, the 3rd and 7th of the original chord are the same pitches as the tritone, only inverted. The two most harmonically important pitches remain the same. Adding the #11 isn't necessary, but allows more reference to the original chord.

The other pitches of the tritone that don't match the original are C and F. Well, C is the b9 which sounds awesome 98% of the time on a dom7 chord, and the F is the tritone. It'll create dissonance, but if you're throwing in a tritone sub, you're already going for it.

Try it out. You'll more often times see it done on dom7 chords mainly because it's easier to pull off. Doing so on a maj7 or min7 is doable, but you have to really be conscience of which notes you're playing, on which beat they're on (up or down beat), and how you resolve each pitch.

I apologize if you already knew this and this came off condescending, but I really like explaining music theory.