Preheat oven to 350° and butter a 9×13-inch baking pan.

In a medium bowl, whisk together the flour, 1 cup of sugar, baking powder, and salt. Add the oil, milk, and eggs. Mix with an electric mixer for 3 minutes. Pour the batter into the prepared pan, then evenly sprinkle the blueberries on top.

In a small bowl, combine the remaining 3 tablespoons of sugar and 1 teaspoon of cinnamon, then sprinkle over the blueberries.

Bake until a cake tester inserted into the center comes out clean, about 35 to 40 minutes.