Preheat your oven to 375°F (190°C). Grease and flour a 9x13-inch baking pan to prepare it for the batter. In a large bowl, sift together 2 ½ cups of flour, 1 ½ cups of sugar, baking powder, and salt. Add the milk, vegetable oil, beaten eggs, and vanilla extract to the dry ingredients, mixing until you achieve a semi-thick batter. In a separate bowl, toss the blueberries with about 2 teaspoons of flour to prevent them from sinking, then gently fold them into the batter. Pour the batter evenly into the prepared baking dish. Bake in the preheated oven for 25 to 30 minutes, or until the cake is lightly golden. Remove the cake briefly, brush the top with melted butter, and sprinkle approximately 2 tablespoons of sugar over it. Return the cake to the oven and bake for an additional 5 to 10 minutes, until the top is golden brown, glazed, and a toothpick inserted into the center comes out clean.