American Flag Cherry Pie

Preheat oven to 425 degrees F with rack in the middle position. Line a rimmed baking sheet with foil. Roll one pie crust to a 13" circle. Transfer to a 9" pie pan. Cut a piece of cardboard to make a 90-degree wedge; wrap in aluminum foil. Arrange wedge in piecrust to shield 1/4 of the pie. Combine cherries, sugar, tapioca, lemon juice, nutmeg, salt, and vanilla in a bowl until well coated. Let stand 15 minutes, then transfer to pie plate. Top with 3 tablespoons butter. Pour canned blueberry pie filling into small, shielded area of the pie crust; top with fresh blueberries. Pour tart cherry filling in large, unshielded area. Top with 3 tablespoons salted butter. Cut 6 stars using a 1" cookie cutter and 7 (3/4") strips from remaining refrigerated pie crust. Remove cardboard shield. Place stars over blueberry filling and strips over cherry filling, trimming as necessary. Fold bottom overhang over strips and crimp to seal. Bake, on prepared baking sheet, until crust is golden brown, 18 to 20 minutes. Reduce oven to 375 degrees F and bake until filling is bubbling, 35 to 40 minutes (cover crust with aluminum foil or a pie shield if it becomes too dark). Cool.