Preheat the oven to 360°F (180°C). Grease a 7 x 11 inch (18 x 28 cm) baking pan well.
Add the cake mix to a large bowl and mix in the butter with your fingertips until it's evenly distributed and small crumbs are forming. Set aside 3/4 cup of the crumbs.
Add the sour cream and eggs to the remaining crumb mixture and beat in with an electric mixer until smooth, about 2-3 minutes. Pour into the prepared pan. Evenly sprinkle first the blueberries and then the crumbs you set aside over the top.
Bake in the preheated oven for 50-60 minutes or until a toothpick inserted near the center comes out clean.
Cool in the pan before drizzling with the glaze (if you want to) and cutting to serve.
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