Combine the blueberries, sugar, salt and 2 cups of water together in a large saucepan. Bring to a boil over medium-high heat and reduce to a simmer. Continue to cook for 5 minutes.
In a small bowl, mix the cornstarch with the remaining 3 tablespoons of cold water. Stir the mixture into the soup and simmer until thickened, about 3 more minutes. Serve the soup warm or chilled.
Notes
Storage Instructions:In the Refrigerator: Store the finished blueberry soup in the refrigerator in an air tight container for up to 4 days. Either warm before serving or serve chilled.In the Freezer: This soup freezes well! Transfer the completely cooled soup to a freezer-safe bag or container and freeze for up to 2 months.