Heat the oven to 350 degrees. Grease the dish. Melt the butter in a medium-size saucepan (or in a bowl in a microwave oven.) Let cool. Wash and drain the blueberries; spread them out on paper towels to dry, removing any bits of leaf or stalk. Put the flour, baking powder, and salt in a large bowl. Stir to mix well. Add the milk, sugar, and eggs to butter. Whisk to blend well. Add to the flour mixture, stirring to blend well. Fold in the blueberries. Spread the batter in the prepared dish. Sprinkle with the sugar-nutmeg topping. Bake until a wooden pick inserted in the center of the cake comes out clean, 50 to 60 minutes. Place the dish on a wire rack to cool at least 30 minutes before serving.