Scrub lemon with a vegetable brush. Using a vegetable peeler, carefully remove only the yellow peel. Juice the lemon. Reserve 2 teaspoons and set aside.

For syrup:

In a small saucepan, combine the water, sugar, lemon peel, and mint. Cook and stir over medium heat until sugar dissolves. Remove from heat; cover and set syrup aside to cool for 15 minutes. Transfer syrup to a medium bowl. Cover and chill for 45 minutes. Remove lemon peel and mint; discard.

In a food processor, combine blueberries and raspberries. Cover and process until smooth. Strain berry mixture through a fine-mesh sieve over the bowl of chilled syrup; discard seeds. Stir in the reserved 2 teaspoons lemon juice.

Pour berry mixture into a 13x9x2-inch baking pan. Cover and freeze for 1 1/2 to 2 hours or until mixture is slushy on the edges. Use a metal spoon to stir, scraping frozen mixture off bottom and sides of pan. Continue freezing about 3 hours more or until all of the mixture is slushy, stirring every 30 minutes. Cover and freeze, without stirring, for 4 to 24 hours or until firm.

To serve, let granita stand at room temperature for 5 to 10 minutes. With the tines of a fork, scrape across the surface of the granita. Spoon into chilled dessert dishes.