Muddle together the mint and sugar in a cocktail shaker. Add the blueberries and lightly muddle. Add the rum, lime juice and some ice to the shaker and shake vigorously. Fill two tall glasses with ice, then pour 3 ounces club soda into each. Divide the rum mixture evenly between the glasses and stir gently. Garnish with a lime wedge, a few blueberries and a sprig of mint, and serve.