In a mixing bowl, whisk together the water and cornstarch until a smooth slurry is formed.
Place a large stock pot over medium/low heat.
Add the fresh blueberries, lemon juice, cinnamon, sugar, and the cornstarch slurry to the stock pot.
Cook the mixture for about 20 minutes, stirring occasionally, until the sauce thickens to a pie-filling consistency.
Once the sauce has thickened, remove from heat and let it cool.
