Preheat oven to 400 degrees. Lightly spoon the flour into measuring cups and level ith with a knife and combine it in a large bowl with the baking powder, salt, sugar, and citrus peel. Make a well in the center of the mixture. In a smaller bowl, whisk together the egg, buttermilk, and oil. Add to the dry ingredients, stirring just until moistened.
In a small bowl, combine the remaining flour and sugar and then toss the blueberries in this mixture until they are well-coated. Gently fold the blueberry mixture into the batter. Spoon the batter into lined muffin tins, filling 2/3 full.
To make the streusel topping, combine the 1/4 c. sugar, 2 1/2 Tbsp. flour, and cinnamon. Cut in the butter with a pastry cutter or two butter knives until the mixture is crumbly. Sprinkle over the batter and bake for 18 minutes or until the tops are golden and a toothpick inserted into the center of one of the muffins comes out clean. Remove from oven and allow to cool in the pan for 5 minutes and then transfer to a cooling rack. Makes 12 muffins.