Blues scale is a pent. scale with that one extra note, flatened 5th. Throw in the 9th, 13th and all this, notes to slide into the main ones, forget the name of them and you've got yourself a pretty versatile scale.

Blues scale involves the flatened 5th which you can't pull out just anywhere but the scale itself is based on the pent. scale which can but used in a lot of circumstances. People will argue that there's better opions than it and to that I've got no comment because it's relative to the tune.