Spread pineapple in the bottom of a greased 9x13" pan. Spread the blueberries evenly over the top of the pineapple. Whisk together the flour and sugar in a small bowl. Sprinkle evenly over the top of the fruit. Sprinkle the dry cake mix over the top. Drizzle with the melted butter.
Bake at 375° for about 40 minutes, or till topping is lightly browned. Serve warm with vanilla ice cream.