In a medium saucepan over medium-high heat, bring blueberries, granulated sugar, cornstarch, and 1/4 cup water to a simmer and cook, breaking up some berries with the back of a spoon, until mixture thickens and berries are tender, about 4 minutes. Remove from heat and stir in lemon juice. Let cool, then cover and refrigerate until cold, about 1 hour.