Preheat oven to 180°C/350F.
Grease a muffin tin line with muffin liners.
To make the streusel topping whisk together sugar and flour. Add the oil and mix till you get a sandy texture. Set aside.
Sift the flour, baking powder and salt and set aside.
Beat butter and sugar in a bowl until light and fluffy.
Stir in eggs, vanilla, and milk.
Add flour mixture and stir until just combined.
Stir in blueberries.
Evenly distribute the batter into 12 muffin cups.
Sprinkle muffins with streusel topping.
Bake for 20 to 25 minutes or until a toothpick inserted into the centre comes out clean.
Cool the muffins for about 5 minutes in the pan before removing to cool on a wire rack.