How to Make It
Preheat oven to 375°F with oven rack in lower third of oven. Roll out 1 piecrust on a work surface into a 14-inch round. Using a 4 1/2-inch round cutter, cut 5 rounds from dough. Repeat procedure with second piecrust. Place jar rings on a baking sheet; place 1 lid inside each with rubber seal facing down. Lightly coat with cooking spray. Place 1 dough round in each prepared lid, pressing into bottoms and up sides of lids; gently flute tops of dough.
Gently toss together peaches, blackberries, and lemon juice in a large bowl. Stir together granulated sugar, cornstarch, and salt in a small bowl. Add sugar mixture to peach mixture; stir gently to coat. Divide mixture evenly among prepared lids. Bake on lower rack in preheated oven until browned and bubbly, 30 to 35 minutes.
Transfer pies to a wire rack, and cool 20 minutes. Remove pies from jar lids. Sprinkle evenly with powdered sugar when ready to serve. Serve warm or at room temperature.