In a medium bowl, combine the blackberries, granulated sugar, and lemon juice. Gently mash the blackberries with a fork until they release their juices. Set aside to macerate for about 15 minutes.
In a separate bowl, whip the heavy cream and vanilla extract using an electric mixer until soft peaks form. Be careful not to overwhip.
Fold the blackberry mixture into the whipped cream gently until just combined, creating a marbled effect.
Spoon the mixture into serving glasses or bowls.
Chill in the refrigerator for at least 1 hour before serving to allow the flavors to meld.
Garnish with mint leaves if desired before serving.