Preheat the oven to 350°F (175°C).
In a medium bowl, combine the fresh blueberries, granulated sugar, and lemon juice. Toss gently to coat and set aside.
In a separate bowl, mix the blueberry muffin mix and milk until just combined.
Pour the melted butter into the bottom of an 8-inch square baking dish.
Spoon the muffin batter evenly over the melted butter in the baking dish. Do not stir.
Carefully spoon the blueberry mixture evenly over the batter. Do not stir.
Bake in the preheated oven for 35 to 40 minutes, or until the top is golden brown and a toothpick inserted into the batter comes out clean.
Remove from the oven and let cool for 10 minutes before serving.