Preheat the oven to 375°F. Prepare a pan by spraying it with cooking spray.
Mix the flour, sugar, and melted butter in a medium bowl. Stir the mixture with a fork until the mixture is crumbly and the size of small peas. Set aside.
Mix the blackberries, sugar, cornstarch, and orange zest in a separate bowl. Stir until combined.
Pour the blackberry mixture into the prepared baking dish and sprinkle evenly with the crumble topping.
Bake the crumble for 45 minutes in the preheated oven. The crumble should be evenly browned on top and bubbly around the edges. You may need to cover the top of the crumble with foil during the last 15 minutes if you find the top is getting too brown.
Let cool slightly before serving with ice cream.
Notes
You can make this in a round dish as long as it is an 8-inch round. The baking time and temperature will be the same.
Place the dish on a baking sheet before putting it in the oven to bake in case the mixture bubbles over.
Oven temperatures vary and may need to be recalibrated periodically to ensure they are accurate. Make sure to check your crumble at the lower end of the recommended baking time.