Boil the water and dissolve the sugar in it. Meanwhile, wash the blackberries and add them to the boiled sugar water solution. Puree everything with a hand blender.
Add the preserving sugar and simmer for 5 minutes, stirring constantly. Leave to cool overnight in the refrigerator. If desired, grate the lemon zest into the mixture. Season the mixture with the juice of the lemon and the orange juice. Beat the egg whites until stiff and fold in.
Place in the ice cream maker for approx. 30 min. Put the sorbet in the freezer for another 3 hours.