Combine blackberries with the sugar in a bowl. Using a fork, smash the berries until they are jam-like and mostly pureed. Stir in the lime zest, lime juice, salt, and creme de cassis if using. Chill in the fridge for 30 minutes.
In the bowl of a stand mixer fitted with a whisk attachment, whip the cream on medium high speed to firm peaks. Fold into the chilled blackberry mixture with a rubber spatula until just incorporated.
Spoon into serving glasses and chill in the fridge for at least 30 minutes. Garnish with additional blackberries and slices of lime, if desired.
Calories: 298kcal, Carbohydrates: 25g, Protein: 2g, Fat: 22g, Saturated Fat: 14g, Cholesterol: 82mg, Sodium: 24mg, Potassium: 149mg, Fiber: 3g, Sugar: 20g, Vitamin A: 990IU, Vitamin C: 17mg, Calcium: 60mg, Iron: 1mg