Use a muddler (or the bottom of the spoon) to crush the mint, lime juice, and blueberry syrup to release the mint oils. Fill the glass almost to the top with ice. Pour the rum over the ice, and fill the glass with club soda or sparkling water. Give it a stir, garnish with fresh blueberries and mint, and enjoy!
Notes
Recipe for quick and easy blueberry syrup: http://www.foodandwine.com/recipes/perfect-blueberry-syrup