Marionberries, an exceptionally juicy, slightly tart blackberry variety developed by Oregon State University, are perfect for this fizzy summer refresher. You can also use regular blackberries.
Combine 1/2 cup mint, sugar, and 4 ounces marionberries in a cocktail shaker; mash with a muddler or wooden spoon until berries are broken down. Add ice cubes, rum, and juices to shaker; seal. Shake vigorously for 10 seconds. Strain mixture evenly into 4 glasses filled with ice; top each with 1/4 cup club soda. Garnish with additional berries and mint, if desired.