1. Preheat oven to 375ºF. Mist an 8-inch square baking pan with cooking spray. Sift flour, baking powder and salt together into a medium bowl.
2. In a bowl, using an electric mixer on medium-high speed, beat butter, sugar and zest until light, about 4 minutes. Add eggs one at a time, scraping down sides of bowl as needed. In another bowl, mix buttermilk and vanilla. Beat a third of flour mixture into butter mixture until nearly combined, then beat in half of buttermilk mixture. Repeat, ending with last third of flour mixture; beat until just combined. Stir in lemon juice.
3. Spread batter evenly in prepared pan and sprinkle blueberries on top, pressing down lightly. Bake until a toothpick inserted into center of cake comes out clean, 50 to 55 minutes. Let cool completely in pan on a wire rack.