1. Combine blackberries and 1/4 cup water in the bowl of a food processor fitted with the metal blade. Process until fruit is very smooth, and pour into a fine mesh sieve set in a deep bowl or a large glass measuring cup. Using a rubber spatula or a wooden spoon, gently press down on the mixture to extract as much liquid as possible. Don't press too hard, or some of the seeds may be forced through the strainer into the liquid. Discard any seeds or pulp that remain in the strainer. Set aside strained puree.

2. In a small bowl, stir 2 teaspoons gelatin into remaining 1/4 cup cold water; let stand until gelatin dissolves, about 5 minutes. Meanwhile, in a small saucepan, heat simple syrup over medium heat just until it is warm to the touch, and remove from heat. Add gelatin mixture, and stir until it is dissolved; let cool. Transfer to a large bowl or plastic storage container, and add reserved fruit puree.

3. In a mixing bowl, beat 2 egg whites until stiff but not dry peaks form, and fold into fruit puree. Cover container, and place in refrigerator until it is well chilled before using, at least 1 hour.

4. Pour mixture into an ice-cream maker, and freeze according to manufacturer's instructions. Transfer sherbet to a large airtight container; place in freezer at least 2 hours or overnight before serving.