Wash blueberries thoroughly.
In a medium saucepan, add the blueberries, water, sweetener and lemon juice.
Cook over medium heat, stirring frequently.
In a small bowl, add the cornstarch and a little water and whisk into a slurry.
When the blueberry mixture starts to bubble, mash some of the blueberries until you get the consistency you want.
Stir in the cornstarch mixture and stir, making sure there are no lumps.
Remove from the heat and allow to cool. (the sauce will thicken when cooling)
Store in a jar or sealed container in the refrigerator.