Preheat oven to 400 degrees. In a large bowl, mix the flour with 1/2 cup sugar, the baking powder, and salt. Make a well in the center. Combine the egg, oil, and milk. Add to the dry ingredients, stir until moistened. Toss the blueberries with 2 tablespoons of sugar and the lemon rind. Fold into the batter. Fill muffin pans lined or greased 2/3 full. Bake for 20 minutes or until golden brown.
While the muffins are cooking, melt the butter and stir in the lemon juice. Dip the warm muffin tops in the butter and sprinkle with sugar. You can use coarse sugar or granulated sugar for the tops.
Make ahead - you can prepare the batter and fill the muffin cups, cover with plastic wrap and refrigerate. Just bake, dip and serve.