Purée blueberries and ginger in a blender or food processor until smooth. 4 cups whole bluebrries should yield 3 cups of purée. Add sugar and blend for 30 seconds. Add lemon and salt, blend to incorporate, and adjust to taste. Strain purée and transfer to refrigerator; chill for 2 to 3 hours until very cold. Churn sorbet in ice cream maker according to manufacturer's instructions. Transfer sorbet to freezer to firm up for 2 to 3 hours before serving.
