Put sugar and cornstarch in a saucepan and mix together with a whisk. This is an easy way to incorporate the cornstarch into the mixture and helps avoid little balls of cornstarch.
Add ½ cup water and turn heat to medium-high.
Add 2 cups blueberries allow to thicken.
Add 1 Tablespoon lemon juice and stir it in.
Let cool at room temperature.
Store in the refrigerator.