Combine sugar, cornstarch and salt in saucepan. Add water and 2 c. blueberries; cook over medium heat, stirring constantly, until mixture comes to a boil and is thickened and clear. (Mixture will be quite thick.)
Remove from heat and stir in butter and lemon juice. Cool.
Place remaining 2 c. raw blueberries in pie shell. Top with cooked berry mixture. C