Puree blackberries in a food processor or blender. Strain through a fine strainer to remove seeds. Soften gelatin in 2 tablespoons cold water. Set aside. In a medium saucepan, place blackberries, sugar and 1/4 cup water. Simmer for 5 minutes. Mix into gelatin mixture. Beat egg whites until stiff but not dry. Fold into berry mixture. Beat cream until stiff peaks form. Fold into berries. Pour mousse into 4 to 6 individual serving glasses, or a decorative bowl. Garnish with fresh blackberries if desired. Chill in refrigerator for 3 hours or until ready to serve.