A correction: what's known as the blues scale is slightly different from the minor pentatonic scale in that it includes the flatted fifth note (in addition to the natural fifth, not instead of it).

So a blues scale in E would be E, G, A, Bb, B, and D; not pentatonic, and not diatonic either.

And upon rereading I realize that my question "how do you decide if you're playing a blues song" probably won't make sense to anybody. So never mind. I'll come back if I can put my finger on what exactly it is that I'm not getting.

Marion