Combine blueberries and sugar in a saucepan with a pinch of salt. Bring to a boil. Mash blueberries with a potato masher or fork. Simmer for 5 minutes, stirring frequently. Pour a little milk into the blender and add the blueberry mixture, then puree until smooth and stir in the cream. Chill it a few hours or overnight, covered, freeze it in an ice cream maker, and serve when firm.