Preheat oven to 375 degrees F (190 degrees C). Grease and flour a 9x13-inch baking dish.
Sift 2 1/2 cups flour, 1 1/2 cup sugar, baking powder, and salt together in a bowl. Add milk, vegetable oil, eggs, and vanilla extract; mix into a semi-thick batter.
Toss blueberries with about 2 teaspoons flour in a separate bowl; fold into batter. Pour batter into prepared baking dish.
Bake in the preheated oven until cake is lightly browned, 25 to 30 minutes. Brush melted butter on top of cake and sprinkle with about 2 tablespoons sugar. Continue to bake until golden brown and a toothpick inserted in the center comes out clean, 5 to 10 more minutes.