In some European countries, fruit soups are served as a dessert, but our guests seem to prefer them as a prelude to dinner.
Blend the cornstarch with a little of the cold water; then add the rest of the water, blueberries, sugar and Marsala wine. Simmer for about 100 minutes, or until the blueberries are soft. Whirl in blender until smooth or put the mixture through a food mill. Cool.
Stir in the sour cream. Serve well-chilled.