In a medium saucepan, bring water to a boil over medium-high heat. Add gunpowder green tea, chopped mint, and sugar, reduce heat to low, and let simmer for 1 minute. Turn off heat and let steep for 15 minutes.
Strain tea through a fine-mesh strainer set over a heatproof container or large spoutd liquid measuring cup or jug, discarding tea leaves and mint.
Allow tea to cool to room temperature, about 1 hour. (To speed this up, place container in a larger bowl filled with ice water for about 25 minutes.)
Once cooled, add orange blossom water, if using. Adjust with more to taste, if desired.
Pour tea into a shallow, freezer-safe 8-by-8-inch baking pan. Freeze until firm, about 1 hour. Use a fork to scrape the mixture, breaking up any ice crystals. Return to freezer and repeat freezing and scraping process every 30 minutes for about 2 to 3 hours(5 to 7 freezing intervals) total, or until the granita is fully frozen into fluffy crystals.
Spoon into serving glasses or bowls and garnish with fresh mint, if desired. Serve immediately,
