It's truly a matter of preference. In this recipe, the apples are baked in the oven for nearly an hour. In that amount of time, they become fork-tender but still retain a bit of body. If you prefer very soft apples, you can always precook the apples with the filling ingredients (minus the blackberries and cornstarch). Cook apples for about 5 minutes in a large pot on the stove over medium heat, stirring frequently.