Preheat oven to 375°F.
Generously butter a 12-muffin tin.
Prepare the crumble topping by using your fingers or a pastry blender to combine the butter, sugar, and flour to form pea-sized bits.
Zest and juice the lemons. Mix the lemon juice with the milk in a liquid measuring cup and let sit. It should equal approximately 1 cup of liquid.
Whisk together all of the dry ingredients with the lemon zest. Pour in the milk mixture, eggs, and melted butter. Gently combine.
Before folding in the blueberries, lightly toss them with flour (this prevents them from sinking). Fold in the berries. Do not over mix.
Spoon the batter into the muffin tins. Evenly sprinkle each muffin with the crumble topping.
Bake 20-25 minutes. Test for doneness with a knife or toothpick. Let cool in the pan for 5 min, then loosen the sides of the muffins by running a knife around each muffin. Let cool for another 10 before taking the muffins out of the tin to finish cooling on a wire rack.