Start by preheating your oven to 400°F (200°C). Grease 8 muffin cups or line them with paper liners.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In a liquid measuring cup, pour in the oil, crack in the egg, and add enough milk to reach the 1-cup mark. Stir until everything is well mixed.
Pour the wet ingredients into the dry ingredients and gently mix just until combined. Don’t overmix. Fold in the blueberries and set the batter aside.
For the crumb topping, add the sugar, flour, butter, and cinnamon to a small bowl. Use a fork to mix until crumbly.
Divide the batter evenly between the muffin cups, filling each one right to the top. Sprinkle the crumb topping generously over each muffin.
Bake for 20 to 25 minutes, until the tops are golden and a toothpick inserted in the center comes out clean. Let cool slightly, then enjoy.