1. In a small saucepan, whisk together the sugar and cornstarch. Add the water and lemon juice, and whisk until smooth. Set the pan over medium heat and bring to a boil, stirring constantly. Let the sauce boil for 30 seconds, then add 1 1/2 cups blueberries. Stir over the heat for 1 minute longer, or until the berries release a little juice but still hold their shape. Transfer to a bowl to cool to room temperature.
2. Spread the blueberry sauce on top of the chilled pie. Sprinkle with the remaining 1/2 cup fresh blueberries and the raspberries. Chill until ready to serve.