Combine ⅔ cup granulated sugar, ⅔ cup water, 1 ½ cups blueberries, and 1 tablespoon lemon zest in a medium saucepan.
Bring to a boil over medium heat, then reduce the heat to medium-low and simmer for 5-10 minutes or until the sugar has dissolved and the blueberries begin to burst.
Remove the pan from the heat and press the blueberry syrup through a fine mesh sieve. Set aside the strained blueberry syrup to cool completely.
Once cooled, add the blueberry syrup, 1 cup freshly squeezed lemon juice,, and 2 cups ice cubes to a blender. Blend until smooth.
Stir in 3 cups sparkling water,, pour into glasses, and serve immediately.