In 1 1/2-quart saucepan, stir sugar, cornstarch and water until smooth. Add 1 cup of the blueberries. Heat to boiling; reduce heat. Simmer uncovered 5 minutes, stirring occasionally. Stir in lemon juice. Cool 15 minutes. Stir in remaining blueberries and the raspberries. Cover and refrigerate at least 1 hour but no longer than 24 hours.