Preheat oven to 350°. In a small saucepan, melt butter; stir in sugar until dissolved. Add 1 teaspoon lemon zest. Pour into a greased 8-in. square baking dish. Arrange blueberries in a single layer over top; set aside. , Prepare cake batter according to package directions. Stir in the remaining lemon zest. Carefully pour over blueberries. , Bake until a toothpick inserted in the center of cake comes out clean, 30-35 minutes. Cool on a wire rack. Serve with whipped cream if desired.