In a bowl, blend together flour, sugar, baking powder, salt, butter or margarine, milk and egg. Using an electric mixer, beat vigorously for 30 seconds. Carefully fold blueberries into batter. Pour into a greased 9-inch-square cake pan.
For topping, in a bowl combine sugar, flour, cinnamon, nutmeg and butter; sprinkle over batter. Bake for 45 to 50 minutes, or until cake tests done.