Cream butter with lemon zest and 1 cup of the sugar until light and fluffy. Add the egg and vanilla and beat until combined. Meanwhile, toss the blueberries with 1/4 cup of 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 buttermilk. Fold in the blueberries.
Spray a 9-inch square baking pan with non-stick spray. Spread batter into pan. Sprinkle batter with remaining tablespoon of sugar.
Bake for 34-45 minutes. Check with a toothpick to see if done.