Fill a pitcher with the ice cubes to chill the base ingredients quickly.
Pour in the dark rum, cranberry juice, blackberry juice, lime juice, and simple syrup.
Stir the mixture well until the simple syrup dissolves and the liquids combine evenly.
Add the club soda and give it one gentle stir to keep the bubbles intact.
Pour into four glasses and drop a few fresh blackberries into each one as garnish.