preheat oven to 350. Cream butter with lemon zest and 1 3/4 cup of sugar until light and fluffy. Add eggs ad vanilla and beat combined. Meanwhile, toss the blueberries with 1/2 cup flour, then whisk together the remaining flour, baking powder and salt. Add the flour mixture to the batter a little at a time, alternating with the butermilk. Fold in the blueberries. Grease 9 in square pan. Spread batter in pan. Sprinkle batter with remaning sugar. Bake 35 min.