Preheat oven to 350 degrees F (175 degrees C). Line 6 jumbo muffin cups with paper liners.
Whisk whole wheat flour, white sugar, baking powder, 1/2 teaspoon cinnamon, and salt together in a bowl. In a separate bowl, whisk skim milk, egg, softened butter, and vegetable oil. Mix the liquid ingredients into the whole wheat flour mixture to make a smooth batter, and gently fold in blueberries. Spoon batter into the prepared muffin cups, filling them to the top.
Combine brown sugar with 1/2 teaspoon cinnamon in a bowl and spoon the mixture onto the muffins, if desired.
Bake in the preheated oven until the muffins are golden brown and a toothpick inserted into the center of a muffin comes out clean, 20 to 25 minutes.
