In a large bowl, beat butter and sugar until crumbly, about 2 minutes. Add egg; beat well. Combine milk and water. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk mixture. Fold in blueberries.
Coat muffin cups with cooking spray or use paper liners; fill three-fourths full with batter. Bake at 350° for 27-30 minutes or until a toothpick comes out clean. Cool in pans for 5 minutes before removing to a wire rack.