Preheat the oven to 450°F (232°C).
In a large mixing bowl, whisk together flour, baking powder, sugar, and salt.
Pour in the buttermilk and stir until just combined. Gently fold in blueberries.
Pour melted butter into a 9x13-inch baking dish. Carefully spoon the dough over the butter, spreading evenly.
Score the dough into 9 squares using a sharp knife.
Bake for 20-25 minutes, until golden brown on top and cooked through.
Allow to cool slightly before serving. Sprinkle with extra sugar if desired.