Mix the cornstarch with the water until well combined. Pour the water mixture, blueberries, sugar, lemon juice, and lemon zest into a pan over medium heat. Cook, stirring occasionally for 4-5 minutes or until thick and some of the blueberries are bursting. Remove from the heat and let cool. Spoon over your waffle, pancake, French toast, crepe, or ice cream. Enjoy.