Blastoise's shell is NOT made of stone, like my knuckles ARE, hence, it ain't a rock type.
& the Darkness is not evil... it only appears that way, due to the fact that the darkness give evil the covers it likes best. In truth, the shadows can be good or evil, just like the Dark-typed pokemons.
In Pokemon mystery dungeon: Explorers of Time/Darkness/Sky, Darkrai was evil, but in the movie, Rise of Darkrai, Darkrai itself played the role of hero, by giving it's life to save the life of the villagers, from what I understand.
Now, back to our discussion on WATER pokemon...
I have a Swampert, & it's pretty reliable.
& I LOVE reliabity!