Preheat the oven to 170 degrees F. Line a baking sheet with parchment, making sure there is an overhang. Spray the parchment with cooking spray.
Place the apples, juice, cinnamon and a pinch of salt into a large saucepan over medium heat. Cook, stirring occasionally, until the apples have softened, 20 to 25 minutes. Remove from the heat and set aside. Pulse the apple mixture in a food processor until smooth and free of any large pieces. Using a ladle, pass the apple puree through a fine-mesh strainer. Set aside.
Meanwhile, place the blueberries, sugar, lemon zest and 1/4 cup water into a small saucepan over medium-high heat. Cook, stirring occasionally, until the berries soften and burst, 8 to 10 minutes. Remove from the heat and crush the berries into a coarse puree using a potato masher. Set aside.
Using an offset spatula, spread the apple puree evenly over the prepared baking sheet. Tap the baking sheet once or twice on the countertop to settle any air bubbles. Using a small spoon, spread the blueberry mixture in thin diagonal lines across the apple puree. Tap the baking sheet again to remove any remaining air bubbles.
Bake, until the surface is smooth and no longer sticky, 3 to 4 hours, then let cool 20 minutes. Remove the parchment from the baking sheet and flip over to peel the paper away from the fruit leather. Slice the fruit leather into 16 long strips, 1-inch wide. Roll each strip in wax paper and store in an airtight container.
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