Put frozen blueberries, granulated sugar, hot water, and lemon juice in a saucepan.
Cook over medium heat for 10-12 minutes until the blueberries start to break down, stirring often. (If you use fresh blueberries it will take less time for them to break down.)
In a small bowl, mix together cold water and cornstarch. Add the mixture to the blueberries in the saucepan while stirring. Stir and cook for 20-30 seconds, until thickened.
Removed from the heat and stir in vanilla and salt.
Store in covered jar in the fridge. Good for up to one week.