Preheat the oven to 350°F and prepare a 9x5-inch loaf pan by lining it with parchment paper or lightly greasing it with butter. In a mixing bowl, combine the flour, baking powder, and salt, then set this dry mixture aside. In the bowl of a stand mixer, beat together the melted butter, sugar, eggs, vanilla extract, lemon zest, and lemon juice until the mixture is smooth and homogenous. Gradually incorporate the dry mixture and milk in two alternating additions—first a portion of the flour mixture, then half the milk, followed by the remainder of each—mixing only until just combined.

If using fresh blueberries, rinse them lightly to leave a trace of moisture, then toss them in a small bowl with 1 tablespoon of flour to lightly coat. This helps suspend them evenly throughout the batter. Fold the flour-coated berries gently into the batter by hand, ensuring they are evenly distributed.

Transfer the batter to the prepared loaf pan and bake for 55 to 65 minutes, or until a toothpick inserted into the center emerges clean. Let the loaf rest in the pan for 30 minutes before transferring to a wire rack set over a tray to catch excess glaze.

To prepare the glaze, whisk together the melted butter, powdered sugar, lemon juice, and vanilla until smooth. Pour it over the loaf, allowing it to set slightly before serving.