1. Place blueberries and half a cup water in a small saucepan over medium heat. Cook until the berries begin to cook and pop. Stir in vanilla extract.
2. In a separate cup or bowl, mix the cornstarch and remaining half cup of water until dissolved. Add to blueberry mixture (it will become cloudy) and stir until it thickens and becomes glossy.
3. Pour in the liqueur and stir well. Set aside to cool.