Actually, depending on the species of turtle, it could be carnivorous, herbivorous or omnivorous.
However, isn't Blastoise a Tortoise? Which would be hervivorous, yes.
True, but not even large herbivores live on berries. It's usually (strangely) a whole lot of leaves.
Though considering that most big pokemon move a little faster than their big animal counterparts, fruit would be helpful for that burst of energy.
Pretty much, all pokemon would have to be able to eat fruit, because upping meowth's beauty metre would be a tough job otherwise. I noticed that even things like staryu, magnemite and banette can be fed berries and fruit products ingame. I'm guessing all pokemon eat the same thing ingame, because giving pokemon specific diets would be irrelevant to the main purpose of the game, thus not worth the money.
In the anime, pokemon usually are shown to eat either fruit, or snacks akin to dog food. It's probably because having pokemon eat other pokemon would be awkward, due to the apparent sentience that is shared between species. Thus, people aren't seen feeding their pokemon what is blatantly the product of another beast. However, offscreen hunting in the wild can be referenced to.
I'm sure if people wanted to flesh out Pokemon to be a more realistic phenomenon, they could make pokemon have more specific diets. But seeing as the game isn't a nature game, but rather a role-playing game about collectible fighting monsters, they don't make eating a big part of it.