Add the blueberries, sugar, and 1 cup of water to a small saucepan.
Cook over medium heat for 8 to 10 minutes, stirring often, until the blueberries burst and the mixture becomes syrupy.
Gently mash the blueberries with a spoon to release more juice.
Remove from the heat and let the blueberry syrup cool for about 10 minutes.
Strain the syrup through a fine mesh sieve into a bowl or jar, pressing lightly on the berries to extract the juice.
Pour the fresh lemon juice into a large pitcher.
Stir in the blueberry syrup until fully combined.
Add 3 cups of cold water and stir well.
Taste and adjust by adding more water for a lighter drink, more lemon juice for extra tartness, or simple syrup for additional sweetness.
Refrigerate for 1 hour until well chilled.
Serve over ice and garnish with lemon slices and extra blueberries if desired.