Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Beat cake mix with 1 cup water, eggs, and vegetable oil until smooth; pour into the prepared baking dish.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 23 to 28 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack. Slice cake in half horizontally to make 2 layers.
Combine white sugar and cornstarch in a saucepan; gradually stir in 1/3 cup water. Add blueberries and bring to a simmer over medium heat. Simmer until thickened, stirring occasionally, about 1 minute. Allow blueberry mixture to cool.
Beat confectioners' sugar and cream cheese in a large bowl until smooth. Stir in vanilla extract and fold in whipped topping.
Place one layer of cake on a large platter; spread blueberry mixture over cake. Arrange remaining cake layer over blueberry filling. Spread cream cheese icing over entire cake.
Add 1 extra tablespoon cornstarch for thicker consistency on the blueberry filling.