In a small bowl, combine the cornstarch with 1 tablespoon of water and lemon juice. Stir until smooth to make a slurry. Set aside.
In a medium saucepan, combine the blueberries, sugar, and remaining water. Heat over medium heat.
Cook, stirring occasionally, until some berries begin to break down, about 3-5 minutes.
Give your cornstarch slurry another quick stir (in case it settles), then slowly pour it into the berry mixture while stirring constantly.
Continue cooking and stirring until the mixture thickens and becomes glossy about 2-3 minutes. Once it bubbles, remove it from heat.
Transfer to a bowl and allow to cool completely.