Preheat oven to 375°F (190°C). Lightly grease an 8x8-inch baking dish.
In a large bowl, gently toss the blueberries with granulated sugar, lemon juice, and cornstarch. Pour the mixture into the prepared baking dish.
In another bowl, whisk together flour, sugar, baking powder, and salt. Stir in the milk and melted butter until just combined. Pour the batter evenly over the blueberries.
Bake for 40 to 45 minutes, or until the topping is golden brown and the filling is bubbling around the edges.
Let cool for at least 10 to 15 minutes before serving.