Make the Blueberry Simple Syrup: In a small saucepan, combine the blueberries, water, and sugar. Bring the mixture to a simmer over medium heat, stirring until the sugar is dissolved and the blueberries have burst. This should take about 5 minutes.

Strain and Cool: Strain the mixture through a fine-mesh sieve, pressing on the blueberries to extract all the liquid. Discard the solids and let the blueberry simple syrup cool.

Mix the Cocktail: Fill a cocktail shaker or a tall glass with ice. Add the blueberry simple syrup and lemonade

Shake and Serve: If using a shaker, shake well until the outside is frosty. If using a glass, stir until all the ingredients are combined. Pour the drink into a glass filled with fresh ice. Garnish with a lemon slice and a few fresh blueberries.