Place a cupcake liner in each cup of a 12-cup muffin pan and set aside.
For the topping, place turbinado sugar, flour, cinnamon and butter in a food processor. Pulse and process until mixture resembles coarse crumbs. Set topping aside.
For the batter, in a medium bowl combine flour, baking powder and salt. In another bowl beat together butter and granulated sugar until creamy. Beat in egg until well-blended. Add flour mixture alternately with buttermilk, mixing well after each addition. Gently fold in blueberries.
Spoon batter evenly into muffin cups and sprinkle with topping. Bake 35 to 38 minutes or until a toothpick inserted into the center of a muffin comes out clean.
Let muffins cool in pan 5 minutes. Transfer to a cooling rack and let cool completely.