Preheat. Preheat oven to 375°F and set aside an 8x8 or 9x9-inch baking dish (or similarly-sized round baking dish).
Prepare the filling. To a large bowl, add all of the fruit and filling ingredients and gently toss until every fruit piece is evenly coated in the sugar, cornstarch, and spice mixture. Transfer the filling to the baking dish and set aside.
Mix the topping. To a separate medium-sized bowl, add all of the crumble topping ingredients. Use a fork, pastry cutter, or clean hands to work the butter into the flour and sugar until the mixture resembles wet sand.
1 cup white whole wheat flour, 1/2 cup light brown sugar, 1/2 cup butter, 1/2 teaspoon cinnamon
Top the crumble. Generously layer the crumble topping over the filling in the prepared baking dish.
Bake. Place the crumble in the oven and bake for 35-40 minutes, or until the crumble is golden brown with bubbling juices and fork-tender apple pieces.
Cool. Cool 10 minutes before serving to allow the juices to thicken more.
Serve. Serve the apple blackberry crumble, topping with ice cream or any other desired toppings. Enjoy!