Prepare Crumble Topping: In a medium bowl, mix together ½ cup of flour, ¼ cup of brown sugar, and 2 tablespoons of white sugar. Drizzle in 3 tablespoons of melted butter, then use a fork to combine until the mixture resembles coarse crumbs. Set aside.

Preheat Oven: Preheat your oven to 350°F (160°C) and prepare a 9-inch loaf pan by greasing it or lining it with parchment paper.

Combine Dry Ingredients: In a large mixing bowl, whisk together 1.5 cups of sifted all-purpose flour, 2 teaspoons of baking powder, and ½ teaspoon of salt.

Coat Blueberries: In a small bowl, toss 2 cups of blueberries with a tablespoon of flour, ensuring they're evenly coated.

Mix Wet Ingredients: In another large bowl, combine ¾ cup of sugar, the zest from 1 lemon, ½ cup of vegetable oil, and the optional teaspoon of lemon extract.

Add Egg and Lemon Juice: Crack in 1 egg and squeeze the juice of 1 whole lemon into the mixture. Stir until smooth.

Combine Wet and Dry Mixtures: Carefully add the dry mixture to the wet ingredients in three additions, gently folding after each addition.

Fold in Sour Cream and Milk: Gently fold in ½ cup of sour cream and ½ cup of milk until just combined.

Incorporate Blueberries: Gently fold the flour-coated blueberries into the batter.

Bake the Loaf: Pour the batter into your prepared loaf pan and sprinkle the crumble topping evenly over the top. Bake for 60 to 80 minutes.

Cool and Drizzle with Glaze: Allow the loaf to cool completely in the pan, then drizzle a glaze made of 3 tablespoons of lemon juice, 1 tablespoon of melted butter, and 1 cup of powdered sugar over it.