Add fresh mint leaves and lime juice to the blender.
Lightly muddle the mint inside the blender jar to release flavor.
Add white rum, blue curaçao, simple syrup, frozen blueberries, and ice cubes.
Blend until smooth and thick, like a frozen cocktail.
Pour into a chilled cocktail glass.
Top with club soda or sparkling water.
Garnish with a mint sprig and lime slice.
Notes
Frozen blueberries help thicken without extra ice. Add more simple syrup for sweetness or more lime juice for tartness. Adjust blue curaçao for deeper color and sweeter flavor. Strain for a smoother texture if desired.