bring 3 cups water to a boil in a large pan. Add blueberries, sugar and cinnamon to taste and cook, stirring frequently, over medium low heat until blueberries have soften, about 15 minutes.
In a small bowl, mix cornstarch with 1/4 cup water and mix until a thick paste forms. Gradually add to the pan, stirring constantly. Bring the soup to a boil over high heat and cook for 3 to 4 minutes, stirring constantly. Remove the pan from the heat and let cool to room temperature.
Pour soup into a blender or food processor and purée. Refrigerate soup, covered, for several hours or overnight before serving.
To serve, ladle soup into chilled bowls. Add 2 tablespoons yogurt or sour cream on top or swirl lightly into the soup. Sprinkle with additional cinnamon, if desired. Makes four servings.