Mix the flour, sugar and baking powder. Mix the oil, egg and milk in another bowl and add to the dry ingredients. Mix well with a spoon. Fold in blueberries (allow frozen ones to thaw beforehand). Line the muffin tin with paper liners and fill in the batter. Fill paper cases full.
Knead the sugar, flour, butter and cinnamon and grate into sprinkles. Put on the dough. The dough should be completely covered with crumble.
Bake for 20-25 minutes.
After a few minutes, carefully remove the finished muffins from the mold and let them cool on a wire rack lined with kitchen paper. They`ll get a little greasy on the bottom.