In a medium saucepan mash about half of the blueberries with a potato masher. Set remaining blueberries aside. Stir sugar, cornstarch, and cinnamon into mashed blueberries. Cook and stir over medium heat until slightly thickened and bubbly. Cook and stir 2 minutes more. Remove from heat. Stir in lemon juice and remaining blueberries. Cover surface with plastic wrap and cool without stirring. Makes 1-1/2 cups.