In a medium saucepan, combine the blueberries, sugar, and water. Bring to a boil over medium heat, stirring occasionally until the sugar dissolves and the blueberries start to break down, about 5-7 minutes.
Remove the saucepan from heat and let the mixture cool for about 10 minutes.
Strain the blueberry mixture through a fine mesh sieve into a bowl or pitcher, pressing down on the solids to extract as much juice as possible. Discard the solids.
Stir in the lemon juice to the blueberry syrup.
To serve, fill a glass with ice cubes, pour in about 2-3 tablespoons of the blueberry syrup, and top with sparkling water or club soda. Stir gently to combine.
Garnish with fresh mint leaves if desired.