Begin by preheating your oven to 375 degrees F (190 degrees C). Prepare a 9x13-inch baking dish by greasing it and dusting it with flour.
In a large mixing bowl, sift together the 2 ½ cups of flour, 1 ½ cups of sugar, baking powder, and salt. Stir in the milk, vegetable oil, beaten eggs, and vanilla extract, blending until you achieve a semi-thick batter.
In a separate bowl, lightly coat the blueberries with approximately 2 teaspoons of flour. Gently fold the flour-coated blueberries into the batter. Pour the batter into the prepared baking dish, spreading it evenly.
Place the dish in the preheated oven and bake until the cake begins to turn a light brown, which should take about 25 to 30 minutes. Remove the cake briefly to brush the top with melted butter and sprinkle with about 2 tablespoons of sugar. Return the cake to the oven and continue baking until it reaches a golden brown color and a toothpick inserted into the center comes out clean, an additional 5 to 10 minutes.