In a small glass bowl, combine the cornstarch and water until you have a smooth paste; slowly pour the cornstarch into the hot blueberry mixture, stirring while you add it, until well combined. Cook 2-3 minutes more, or until the mixture is thick, dark, and clear. Remove from heat. Pour the finished filling into a heat-proof glass bowl, and cover with plastic wrap set directly on top of the filling to cool.* Cool to room temperature, or place it in the fridge to use the next day.
Pour the cooled mixture into a prepared pie shell or spoon into baked tart shells.
*If you want to can this pie filling for a later date, ladle the filling into prepared canning jars while the filling is still very hot.