In a cocktail shaker, combine the blackberries, mint leaves, simple syrup, and lemon juice. Muddle until the blackberries are crushed and the mint leaves are bruised.

Add the bourbon to the shaker and fill with ice.

Shake until well chilled.

Strain the cocktail into a rocks glass filled with ice. Top with club soda if desired. Garnish with a mint sprig and a blackberry. Serve immediately.