Combine dry ingredients (flour-salt) in a bowl.
Add the milk and oil and stir until combined. Gently stir in the blueberries.
Grease a 10-12 oz mug and transfer the batter into the mug. Sprinkle the top with the coarse sugar, if using it. Microwave the muffin for 75-90 seconds, or until the top is set and springs back gently when pressed.
Let the muffin cool for two or three minutes, and enjoy.
*To make this muffin whole grain substitute three tablespoons whole wheat pastry flour for the regular flour.
** To make this muffin vegan substitute coconut milk or your favorite non-dairy milk for the milk.