In a collins glass or highball glass, add the blackberries, mint leaves and simple syrup.
Muddle to release the oils of the mint leaves and the juices of the berries. (See note for an alternate method.)
Top with ice cubes, filling the glass all the way.
Add the lime juice and rum. Top with club soda.
Garnish with a straw, lime wedge and a mint sprig.
Notes
Use fresh berries if you can, especially when they are at their peak In the winter, frozen berries will carry the best flavor. If you don’t have a muddler, use the blunt end of a wooden spoon. If you don’t like chunks of fruit in your cocktail, make the drink in a shaker or mixing glass (except the club soda) then strain into your drinking glass and top with club soda.