Prepare the Crumble Topping by combining flour, brown sugar, and granulated sugar. Mix with melted butter until it resembles wet sand.

Preheat your oven to 350°F (160°C) and grease a 9-inch loaf pan.

In a large mixing bowl, whisk together sifted all-purpose flour, baking powder, and salt.

Toss the blueberries with a dusting of flour to prevent them from sinking during baking.

Blend granulated sugar, lemon zest, vegetable oil, and lemon extract using an electric mixer.

Add egg and lemon juice, mixing until smooth and creamy.

Gradually add the flour mixture, folding gently until no dry flour remains.

Stir in sour cream and milk until all ingredients are well combined.

Fold the floured blueberries into the batter, ensuring even distribution.

Transfer the batter to the prepared loaf pan, sprinkle with crumble topping, and smooth the top with a spatula.

Bake for 60-80 minutes, or until a toothpick inserted comes out clean.

Let cool for 15 minutes in the pan, then transfer to a wire rack to cool completely. Drizzle with glaze before serving.