Add the blueberries to a lowball glass and crush with a muddler. Add the Fresh Mint Syrup and fill the glass with ice. Top with the sparkling lemonade and stir everything together with a spoon. Thread 3 blueberries and the mint sprig on a small skewer and set on the side of the glass for garnish. Serve immediately.
Fresh Mint Syrup:
Stir together the sugar and 3/4 cup water in a small saucepan. Bring to a simmer over medium heat and cook until the sugar completely dissolves, 3 to 5 minutes. Turn off the heat. Use your hands to tear the bunch of mint in half and add it to the saucepan. Using a spoon, press down the mint to make sure that it is completely submerged. Allow the syrup to cool to room temperature in the saucepan, about 1 hour. Strain the mixture through a fine-mesh strainer set over a small bowl or liquid measuring cup. Use the back of a spoon to press down on the mint to extract as much flavor as possible. The syrup can be used immediately and will keep in the refrigerator for up to 2 weeks.