Preheat oven to 375°F (190°C). Butter a 13 x 9-inch (3 L) glass baking dish.
In prepared baking dish, combine apples, blueberries, sugar and flour; spread evenly.
Topping
In a bowl, whisk together sugar, milk, butter and vanilla. Without stirring, sprinkle with all-purpose flour, whole wheat flour, baking powder, cinnamon and salt. Stir with a fork, just until moistened.
Drop topping by 6 large spoonfuls over fruit, spreading gently to cover.
Bake for about 40 min or until fruit is bubbling, topping is golden and tester inserted in centre of topping comes out clean. Let stand for at least 5 min before serving. Serve hot or warm.