Place blueberries, sugar, lemon zest, lemon juice, water, and cornstarch in a sauce pan and stir together. Bring the mixture to a boil, then reduce heat and continue to simmer for 5-10 minutes, or until mixture is slightly thickened and blueberries have begun to break down. I prefer to leave my blueberries whole, but you can feel free to smash them at this point.