Sort the blackberries and wash if necessary. Puree 400 g berries, 2 tablespoons orange juice and zest with a hand blender and pass through a sieve. Stir in sugar.
Soak gelatine in cold water. Heat the rest of the juice in a small pot. Squeeze the gelatine. Dissolve in the juice while stirring. First stir 2 tbsp. blackberry mixture into the gelatine, then stir the gelatine mix into the remaining blackberry mixture.
Chill for about 30 minutes until the mixture begins to gel. Whip the cream until stiff. Fold into the blackberry mixture. Pour the mousse into six glasses, chill for about 1 hour. Decorate with the rest of the berries.