In a mixing bowl, combine flour, sugar and baking powder. Add egg, milk, butter and lemon peel; 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 degrees F (176 degrees Celsius) for 40-45 minutes or until cake tests done.