In a bowl, combine flour, sugar and baking powder. Add egg, milk, butter and lemon zest; mix just until dry ingredients are moistened. Fold in the blueberries. Spread in a greased 9-in. square baking pan. For topping, combine sugar, flour, walnuts and cinnamon. Cut in butter until mixture is crumbly. Sprinkle over batter. Bake at 350° for 40-45 minutes or until a toothpick inserted in center comes out clean.