1. Bring water, sugar, mint, and lemon peel to a boil in a medium saucepan over medium heat. Reduce heat and simmer until sugar is completely dissolved. Remove from heat; cover and let stand 15 minutes. Transfer syrup to medium bowl and refrigerate until cool, 45 minutes. Discard mint and lemon peel.
2. Meanwhile, place a metal 13x9-inch baking pan and a metal spoon in the freezer.
3. Puree blueberries and raspberries in a food processor until smooth. Strain berry puree through a fine sieve over bowl of syrup. Add the lemon juice and stir until combined. Pour mixture into chilled pan.
4. Freeze berry mixture until ice crystals form around edges of pan, about 30 minutes. With the chilled spoon, scrape the ice crystals from edges and stir into the mixture. freeze mixture 1 to 1-1/2 hours more, stirring every 30 minutes, until all of the liquid is frozen. For a fluffy consistency, stir granita mixture with tines of fork just before serving. Makes 4 cups (8, 1/2-cup servings).