How to Make It
Preheat oven to 375°.
Weigh or lightly spoon 9 ounces flour into dry measuring cups; level with a knife. Combine 9 ounces flour and next 3 ingredients (through salt), stirring with a whisk. Remove 2 tablespoons flour mixture; sprinkle over blueberries, tossing to coat.
Combine brown sugar and next 4 ingredients (through cinnamon) in a medium bowl. Drizzle with 2 tablespoons melted butter; toss to combine.
Place granulated sugar, 1/4 cup butter, and oil in a large bowl; beat with a mixer at medium speed until blended (about 2 minutes). Add egg; beat well. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture; mix after each addition. Stir in rind and juice.
Spoon half of batter into a 9-inch square light-colored metal baking pan coated with cooking spray (do not use a dark or nonstick pan). Sprinkle evenly with 1 cup blueberry mixture. Spoon remaining batter over blueberries. Sprinkle evenly with remaining 1 cup blueberry mixture. Sprinkle with brown sugar mixture. Bake at 375° for 45 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely on a wire rack.