In a large saucepan, combine the cornstarch and water until smooth. Stir in blueberries, sugar and lemon juice. Bring to a boil; cook and stir for 1 minute or until thickened. Cool slightly. Transfer mixture to a blender; cover and process until smooth. Strain and discard seeds. Pour sauce into a bowl; cover and refrigerate until chilled.
To serve, spoon sauce over scoops of sorbet. Garnish with additional blueberries if desired. Refrigerate leftover sauce.