In a saucepan bring blueberries, sugar, and salt to a boil over moderate heat, mashing berries and stirring with a fork. Simmer mixture, stirring frequently, 5 minutes and cool slightly. In a blender purée mixture with milk just until smooth and stir in cream. Pour purée through a sieve into a bowl, pressing on solids with back of a spoon. Chill mixture, covered, at least 2 hours, or until cold, and up to 1 day.
