For the simple syrup: Heat the sugar and 1 cup water in a medium saucepan over medium heat, stirring occasionally, until the sugar is dissolved. Remove from the heat and let cool.
Juice the lemons; you should have about 1½ cups lemon juice. Place the blueberries in a pitcher and muddle with a potato masher or the handle of a wooden spoon. Add ice, then add the lemon juice, 1 cup of the simple syrup and 5 cups cold water. Stir to combine.
Pour the lemonade into glasses and top with sparkling water. Add more blueberries.