Combine frozen blueberries, sugar, and lemon juice in a small saucepan over medium heat. Cook for 5-8 minutes, stirring often, until berries thaw and release their juices.

Mix water and cornstarch in a tiny bowl to create a slurry. Once berries are simmering and have broken down, stir in the cornstarch mixture.

Continue cooking and stirring until the sauce bubbles and thickens. Let it bubble for about 15 seconds.

Immediately pour the sauce into a clean bowl. Let it cool completely at room temperature, then refrigerate for at least 1 hour until thoroughly chilled and set.