Let the blackberries, sugar and lemon juice simmer gently for 10 minutes until tender. Brush through a sieve to remove the seeds. Reserve 1/3 of the puree obtained.
Heat gelatin in water while stirring and dissolve, then add to the other 2/3 of the fruit puree. Let cool down until the mass starts to gel.
Whip the crème double, then whip the egg whites. Fold both under the gelling fruit mixture. Divide between 6 molds and refrigerate.
To serve, place 1 tablespoon each of the reserved fruit puree on a plate and top it with the mousse. Decorate with blackberries.