Place blueberries and mint leaves in the cocktail shaker.
Muddle gently until blueberries release their juice and mint becomes fragrant.
Add rum, lime juice, and simple syrup to the shaker.
Fill shaker with ice and shake for 10-15 seconds.
Strain mixture through a fine-mesh strainer into a tall glass filled with ice.
Top with club soda and stir gently with a bar spoon.
Garnish with fresh blueberries and mint leaves.
Notes
Store fresh blueberries in the fridge to keep them firm for muddling.Make simple syrup by mixing equal parts sugar and water in a pan. Heat until sugar dissolves, then cool before using.For a mocktail version, replace rum with coconut water or white grape juice.Frozen blueberries work too - let them thaw slightly before muddling.The drink tastes best when served right away while the mint is fresh and the club soda is fizzy.