For the blueberry syrup: Put the blueberries, sugar and the juice from one lemon n a saucepan. Bring to a boil, stirring constantly, then reduce the heat and simmer until thick, 20 to 25 minutes.
Strain the blueberries through a fine-mesh strainer, pressing to get all the liquid out, then place the liquid in the fridge and chill until cold.
For the lemonade: Mix the sugar with 6 cups of tap water and stir until the sugar is dissolved to make a simple syrup.
Pour the juice from the remaining 24 lemon into a large vat.
Pour in three-quarters of the simple syrup and 8 cups of water.
Add the blueberry syrup, stir it around and then give it a taste.
Add more simple syrup if it needs sweetness, or more water if it needs dilution. Top up with lots of ice before serving.