Wash mint, shake dry, pluck leaves from the stalk and chop finely. Sort the blackberries. Mash 400 g blackberries with mint, orange juice and zest with a fork and pass through a sieve.
Stir in sugar.
Soak gelatine in cold water. Warm the liqueur lukewarm. Squeeze the gelatine and dissolve in it. Stir into the blackberry mixture, chill for approx. 30 minutes, the mixture begins to gel.
Whip the cream until stiff. Fold into 2 portions under the blackberry mixture. Pour the mousse into 6 glasses, chill for about 1 hour. Decorate with remaining blackberries.