Whisk together the balsamic vinegar and corn starch in a medium saucepan. Add the blueberries, corn syrup and salt. Turn the burner to medium heat until boiling, smashing the blueberries as you stir. Reduce the heat to medium-low and stir occasionally for 3 minutes. Pour the sauce through a sieve to remove the blueberry pulp and serve warm. Makes 2 cups.