Special equipment: 2 ice cube trays
Combine the sugar with 6 cups water in a medium saucepan and bring just to a simmer over medium-low heat, stirring until the sugar is dissolved. Turn off the heat and cool to room temperature, then stir in the lemon juice.
Add a few blueberries to each compartment of 2 ice cube trays and cover with lemonade (you will use about half the total amount). Freeze completely, about 4 hours. Refrigerate the remaining lemonade.
Combine the blueberry lemonade ice cubes and the cold lemonade in a blender and blend until smooth. Serve immediately.