Rinse berries and remove any stems. Dry on paper towels.
In sauce pan, stir together potato starch and cold water until dissolved. Add boiling water and stir until smooth. Add sugar, salt, and 1/2 cup of blueberries.
Heat over medium heat, stirring constantly. While heating, crush berries with fork. Cook until mixture boils and becomes somewhat clear (about 10 minutes). Reduce heat to low and cook for 3 more minutes.
Remove from heat, and stir in lemon and butter. Then stir in blueberries, combining gently.
Pour into crust and chill for 3 hours.