If using frozen blueberries, add them to a microwave safe bowl and microwave for 30-45 seconds until blueberries are thawed. Divide blueberries between 2 pint-size highball glasses.
Add lime wedges, simple syrup, and mint leaves to glasses and muddle lightly until lime wedges are juiced and mint leaves are fragrant.
Add rum to the muddled mixture, then fill glass with ice, top with soda water and stir before serving. If desired, garnish with additional lime wedges and fresh mint.