Pre-heat the oven to 425 degrees. In a bowl, combine the apples, blueberries, sugar, flour, lemon juice, cinnamon and salt and let stand for 15 minutes, stirring occasionally.
Place one piecrust in the bottom of a deep-dish pie pan. Pour the apple-blueberry mixture into the pie pan. Dot the top with butter and cover with the other piecrust, sealing edges.
Cut steam holes in top, place on cookie sheet pan and place in oven and bake for 40 minutes.
Reduce the temperature to 350 degrees and bake for an additional 40 minutes. Fruit juices should be bubbling though the vent holes. Stick the point of a knife through the hole and check to see if apples are tender.
Allow pie to cool for 4 hours before slicing.
Makes 8 servings