Preheat the oven to 350 degrees Fahrenheit.
In a small bowl, combine all ingredients for the crumble together and mix until well combined. Set aside.
In a medium mixing bowl, stir together all ingredients for the filling.
Transfer to a 9 inch cast iron pan or baking dish.
Sprinkle on the crumble topping. It will seem like a lot at this point, but use it all! A lot of it gets absorbed into the blueberries while baking, especially if you use frozen blueberries.
Bake for 45-55 minutes or until the blueberries are nice and bubbly and the topping is golden brown. If using frozen berries, bake time will be 50-60 minutes.
Let cool in the pan for about 10 minutes (it will thicken mostly as it cools), then serve with vanilla ice cream or whipped cream.