Soak the gelatin in cold water. Rinse and sort the blackberries. Boil 3 tablespoons of water, lime juice, and sugar in a saucepan, then reduce the heat and simmer for about 8 minutes. Strain through a sieve. Squeeze out the gelatin, stir into the blackberry puree and dissolve. Refrigerate for about 1- 1 1/2 hours, until the mixture starts to gel (occasionally stir and check that the mixture is not too stiff).
Whip the cream until stiff with the lime zest. Beat the egg whites until stiff. Fold both into the gelatin. Spoon into dessert bowls and refrigerate for 3-4 hours, then serve.