
In a small saucepan, combine the blueberries, granulated sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the blueberries soften and release their juices, about 5 minutes.
In a small bowl, mix the cornstarch and water to make a slurry. Stir the slurry into the blueberry mixture. Reduce to a simmer and cook until the sauce has slightly thickened, about 2-3 minutes.
Remove from heat and stir in the lemon zest. Serve immediately, or cool to room temperature, then place in an airtight container in the fridge for up to 1 week. Sauce will thicken in the fridge, so reheat gently in the microwave or on the stove to thin out.