In a saucepan, combine blueberries, sugar, and 1 cup water. Simmer for 8–10 minutes until berries burst.
Strain mixture through a fine mesh sieve, pressing to extract all juice. This is your blueberry syrup.
In a large pitcher, mix syrup with fresh lemon juice and 4 cups cold water.
Stir well, taste, and adjust sweetness by adding more sugar or water if needed.
Serve over ice, garnished with lemon slices, blueberries, and mint sprigs.