Combine flour, brown sugar, white sugar, and cinnamon.
Add softened butter.
Mix topping ingredients by combining them in a medium bowl and pressing them together with your fingers. Ensure no flour remains and all the butter has been absorbed into the flour and sugar mixture. You will have large, heavy crumbs, more like chunks.
Put the bowl with the topping mixture into the freezer while you prepare the fruit filling.
Prepare the Fruit Topping and Bake the Crumble
While the topping is chilling, combine the diced apples and blueberries.
Sprinkle cornstarch and sugar over the apples and blueberries.
Place the fruit in an even layer into an 8" by 8" square baking dish or equivalent baking dish or baking pan.
Evenly distribute the chilled topping over the fruit mixture.
Bake for 45 - 50 minutes, until the crumble topping is browned and juices are bubbling up on the side.
Let the crumble sit for a few minutes before serving.
I prefer my crumble just plain, but my mom likes vanilla ice cream or whipped cream on hers.