Pentatonic scales have five and only five notes--but that is not the only defining element. Pentatonic melodies don't shift to dominant harmony, which is to say that they don't have any notes in them that require a chord change, so you don't feel that intuitive chord shift(though it is possible to use diatonic chord changes in a pentatonic melody, you just don't have to)--

I have commented on Blues before, and my opinion(which doesn't count for much around here, I know) is that it isn't pentatonic--when you play a twelve bar blues, you actually use more notes than are in a diatonic scale-that said, which ought to rule it out. However, you can use a pentatonic scale to play over a blues progression (In again) but you can also play note that are not in the pentanic scale (out). I also refuse to believe that any scale that include the 4th, the b5, and the 5th, is pentatonic.

If you suspect that a melody is pentatonic, try to play it on the black keys(as mentioned above)--if you can't. it isn't--