Put the blackberries in a small bowl and mash lightly with a fork.
Mix in the yoghurt and powdered sugar, fold in the lemon peel that has not been grated.
If desired, stir in a little more powdered sugar (the mixture may still appear a little too sweet when left unrefrigerated, as this taste is somewhat lost when it is frozen).
Freeze in the ice cream maker, best enjoyed immediately.
Garnish with fresh blackberries, possibly marinated with liqueur, and mint or lemon balm.