In the bowl of an electric mixer fitted with the paddle attachment, beat the melted butter, sugar, eggs, vanilla extract, lemon juice, and lemon zest together until combined about 2-3 minutes. In a separate bowl, combine the flour, baking powder, and salt. Stir the flour mixture into the egg mixture alternating with the buttermilk, starting and ending with the flour mixture. In a separate bowl, toss the blueberries with 1 tablespoon of flour, fold into the batter. Pour the batter into prepared loaf pan.