Preheat your oven to 375°F (190°C). Lightly grease and flour a 9x13-inch baking dish to prevent the cake from sticking.
In a large mixing bowl, sift together 2 ½ cups of all-purpose flour, 1 ½ cups of granulated sugar, the baking powder, and salt. Add the milk, vegetable oil, beaten eggs, and vanilla extract. Mix everything together until a semi-thick batter forms.
In a separate small bowl, toss the fresh blueberries with about 2 teaspoons of flour. This helps keep them from sinking during baking. Gently fold the floured blueberries into the batter.
Pour the batter evenly into the prepared baking dish. Place it in the preheated oven and bake for 25 to 30 minutes, or until the cake is lightly browned on top.
Remove the cake briefly from the oven, brush the melted butter evenly over the top, and sprinkle with about 2 tablespoons of sugar (or more, to taste). Return the cake to the oven and bake for an additional 5 to 10 minutes, or until the top is golden brown, slightly glazed, and a toothpick inserted into the center comes out clean.