Preheat the oven to 200C/180C Fan/Gas 6. Line an 8" square tin.
In a large bowl, combine the flour, baking powder and sugar. Make a well in the centre and add in the milk, butter, egg and oil. Stir with a wooden spoon until just combined.
Sprinkle the cornflour over the blueberries to coat them. Stir gently into the batter. Pour batter into lined tin.
In a small bowl, combine the butter, oats, sugar and flour for the streusel topping. Sprinkle this in an even layer over the muffin batter. Bake for 25-30 minutes, until the top is a light golden colour. Check that a skewer inserted comes out clean. Let cool on a wire rack before slicing and serving.