Put the gelatin sheets in a flat dish of cold water. Leave to soak.
Rinse and drain the blackberries, put aside a few for decoration. Put the rest in a saucepan and add the apple juice. Cook for about 5 minutes then add the sugar and lemon juice. Cook for another 5 minutes or so until the berries are tender. Taste and adjust the lemon juice and sugar content to your liking.
Turn off the heat. Using a hand-held blender process until you get a thick purée.
Squeeze the water out of the gelatin sheets, then add them to the hot purée. Stir until dissolved.
Let the purée cool, then put it in the refrigerator for about 30 minutes until it's about to set.
Whip the cream until you get light foams then gently fold into the purée. Spoon the mousse into small glasses or pots and chill until set. Decorate each pot with 1-2 blackberries and a pinch of lemon zest just before serving.