In a large microwave-safe mug, combine flour, granulated sugar, brown sugar, baking powder, salt, and cinnamon. Mix well.
Add butter to the mug. Use your fingers to rub the butter into the dry ingredients until the mixture is coarse.
Stir in the milk until the mixture forms a thick batter. If too thick, mix in another splash of milk. Gently fold in the blueberries.
Microwave on high for about 1 minute to 1 minute and 45 seconds, or until the muffin is set. Cooking time will vary depending on your microwave and mug size. Enjoy warm.