Preheat the oven to 350°F (175°C).
In a medium bowl, combine the fresh blueberries, granulated sugar, lemon juice, and cornstarch. Stir gently until the blueberries are evenly coated.
Pour the blueberry mixture into a 9-inch round baking dish or similar sized ovenproof dish.
In a separate bowl, mix the blueberry muffin mix, milk, and melted butter until just combined to form the batter.
Drop spoonfuls of the batter evenly over the blueberry mixture in the baking dish. Do not spread; the batter will spread during baking.
Bake in the preheated oven for 30 to 35 minutes, or until the topping is golden brown and a toothpick inserted into the topping comes out clean.
Remove from the oven and allow to cool slightly before serving.