Blues scales are used for improvisation, which is something that doesn't happen in Western Classical Music, and so Common Classical Music Theory isn't very helpful for defining them, as was pointed out above.

When the idea of "pentatonic scale" is used in connection with blues, rather than meaning that there are just five notes, it is a way of trying to explain that there is no shift of tonal center like there is in diatonic music--which is to say that the solo sounds like is is happening within one chord, never mind the odd bent sounds tossed in here or there--

Of course, it doesn't always work like that--