In a medium saucepan, combine the fresh blueberries, 1 cup of water, and the granulated sugar. Place the saucepan over medium heat and bring the mixture to a gentle simmer, stirring occasionally. Continue simmering until the sugar fully dissolves and the blueberries begin to burst, which should take about 5-7 minutes.
Once the blueberries have burst, remove the saucepan from the heat. Allow the mixture to cool slightly for a few minutes.
After cooling, strain the blueberry mixture through a fine mesh sieve into a bowl or pitcher. Using the back of a spoon, press down on the solids to extract as much delicious juice as possible. Discard the solid remnants and set the vibrant blueberry syrup aside.
In a blender, add the fresh basil leaves along with 1 cup of water. Blend on high speed until the basil is finely chopped and the mixture is homogenous.
Strain the basil mixture through a fine mesh sieve to collect the liquid, discarding any remaining solids in the sieve.
In a large pitcher, combine the freshly squeezed lemon juice, the blueberry syrup, the basil liquid, and the remaining 3 cups of water. Stir the mixture thoroughly to ensure everything is well incorporated. Taste the lemonade and adjust sweetness as desired—add more sugar for sweetness or additional water to dilute.
Cover the pitcher and refrigerate the blueberry basil lemonade for at least 1 hour to allow it to chill and the flavors to meld beautifully.
To serve, fill individual glasses with ice cubes, then pour the chilled lemonade over the ice. Garnish each glass with a slice of lemon and a sprig of fresh basil for an elegant touch.
Notes
Adjust sweetness to taste and serve chilled for the best flavor.