In a small bowl, cream butter and sugar. Add egg and lemon zest; beat well. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk. Fold in blueberries. Pour into a greased 5-3/4x3x2-in. loaf pan.
Bake at 350° for 45-50 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack. Combine topping ingredients; spread over warm bread.