In a small bowl, beat the cream cheese, milk and extract until smooth. Spread over the bottom of pastry shell.
In a large saucepan, combine the cornstarch, water and lemon juice until smooth. Mash 2 cups blueberries and add to the pan. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Remove from the heat; cool for 15 minutes.
Stir in sugar substitute. Spoon over cream cheese layer. Top with the remaining blueberries. Refrigerate for 3 hours or until set. Refrigerate leftovers.