Preheat the oven to 350°F. Spray 13 x 9-inch baking pan with non-stick cooking spray.
Spread blueberries in prepared pan. Sprinkle with sugar and 1/2 teaspoon cinnamon.; toss to coat.
Top with cake min, spreading evenly. Top with butter slices in a single layer, covering cake mix as much as possible. Sprinkle with remaining 1/4 teaspoon cinnamon.
Bake 50 to 60 minutes or until toothpick inserted into center of cake comes out clean. Cool at least 15 minutes before serving.