After playing some blues licks or solos that requires a lot of bending my strings go out of tune, sometimes it may be just a little bit flat and some other times it can go really flat.

Is it normal the strings to go a little flat after a lot of bendings?
How blues guitarists can do all these bendings and not to go out of tune? BB King is bending all the time and you will never hear him out of tune, how does he do it?