Preheat oven to 350 degrees.
Cream butter with lemon zest and 7/8 cup minus 1 Tablespoon of sugar until light and fluffy.
Add the egg and vanilla and beat until combined. Meanwhile, toss the blueberries with 1/4 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 blueberries.
Grease a 9-inch square baking pan with butter. spread batter into pan. Sprinkle batter with remaining tablespoon of sugar.
Bake for 35 minutes. Check with a toothpick for doneness. If necessary, return pan to oven for a couple of more minutes.
(Note: Baking for as long as 10 minutes more might be necessary.)
Let cool at least 15 minutes before serving.