Place frozen blueberries in a medium saucepan, and bring to a boil. Add sugar and stir until dissolved. Reduce to a rapid simmer and allow the blueberry mixture to thicken for 3 to 5 minutes. In a small bowl or cup, dilute cornstarch with a little bit of water (just enough to dissolve), and add to the blueberries. The sauce will quickly thicken. Remove from heat and cool.