Arrange a rack in the middle of the oven and heat the oven to 350°F. Lightly coat a 9×13-inch baking dish with unsalted butter.
Layer the following in the baking dish in the following order (do not stir together): Spread 2 (about 21-ounce) cans blueberry pie filling into an even layer. Sprinkle evenly with 1 (15.25-ounce) box yellow, white, or lemon cake mix. Cut 1 1/2 sticks unsalted butter into thin slices and arrange over the cake mix in an even layer, covering the entire surface. Scatter 2 cups fresh or frozen blueberries on top in an even layer.
Bake until golden brown, bubbling, and a crispy crust begins to form, 60 to 65 minutes. Serve warm with vanilla ice cream if desired.