Preheat the oven to 425°F (218°C) and place the oven rack in the center position. Line a 9" x 5" loaf pan with parchment paper.
For the crumb topping, place 1.5 tablespoons butter, ¼ cup granulated sugar, and ¼ cup all-purpose flour in a bowl and use a fork to mix it until it becomes a coarse crumble. Set aside.
In a large bowl, whisk together 2 cups all-purpose flour, 2 teaspoons baking powder, ½ teaspoon salt, and 1 tablespoon corn starch.
In a medium bowl, whisk together 1 cup granulated sugar, 8 tablespoons unsalted butter melted, zest of half a lemon, ½ cup sourdough starter discard, 2 large eggs, 3 tablespoons sour cream, ¼ cup milk, and 1 teaspoon vanilla extract until fully combined. Add to the dry ingredients and fold in 1 cup blueberries.
Pour the batter into the loaf pan and sprinkle the crumb topping over the top. Lower the oven temperature to 350°F (176°C) and bake the loaf for 45-60 minutes. Check for doneness with a toothpick around 45 minutes. If the top is browning too quickly, tint with aluminum foil and continue baking until a toothpick comes out clean.
Remove the loaf from the oven and allow it to cool completely in the loaf pan before slicing.