In a food processor, puree watermelon, working in 2 batches. Strain puree through a sieve, reserving liquid; discard solids. In a 9" x 13" metal pan, combine watermelon puree, juice and agave. Freeze, scraping thoroughly with a fork every 20 to 30 minutes, until granita resembles coarse crystals, 2 to 4 hours. Remove from freezer; scrape granita once more with a fork. Divide among 6 glasses. Top each with 1/3 cup blueberries and a mint sprig, if desired.
Image Credit: Levi Brown
Recipe Courtesy of: Adeena Sussman