In a tall 16-ounce glass, muddle mint leaves, lime juice and sugar. Add 2 tablespoons of the blueberries and mash until juicy. Fill the glass three-quarters with ice. Add lemonade until the glass is two-thirds full. Add splash of soda, light rum and remaining blueberries; stir. Slowly pour dark rum into the drink so it floats on top. Garnish with a sprig of mint and slice of lime; serve immediately.