Preheat oven to 350 if using a glass cake pan or a shiny cake pan. (Preheat oven to 325 if using a dark metal cake pan.)
In a large mixing bowl, combine cake mix, eggs, milk, and melted butter.
Mix ingredients until well combined and most bubbles are gone.
In a small saucepan, combine brown sugar and butter.
Simmer on low until the mixture is melted and well-blended. The mixture should resemble a thick caramel.
Spray two nine-inch round cake pans with Pam.
Gently pour the brown sugar mixture into the cake pans, dividing in half.
Carefully spread one cup of blueberries over the caramel mixture in each pan.
Next, gently pour the cake batter over the blueberries, again dividing in half.
Bake according to cake mix box instructions (typically 27 minutes to 35 minutes). Bake until sides pull in, cake top springs easily, and a toothpick in center comes out clean.
Let the cakes cool in their pans on the stove for about 15 minutes.
Gently, flip the cake onto a flat round platter by placing the platter on top of the cake pan and then carefully turning it all over. The blueberries should now be on top.
Allow the cake to set on the counter with the cake pan still on top for 10-15 minutes longer to help ease the topping onto the cake.
Allow cake to cool completely before serving.