In a large saucepan over medium heat, combine blueberries, water and sugar. Stir frequently, but careful not to crush the berries, bring to a low boil.
In a small bowl, mix the cornstarch with cold water until combined.
Slowly stir the cornstarch slurry into the blueberries, careful not to crush them. Simmer until the homemade blueberry sauce is thick enough to coat the back of a metal spoon, about 10 minutes.
Remove from heat and gently stir in vanilla.
