In a large mixing bowl, combine 5 cups of peeled and sliced peaches with 3 cups of blueberries. Sprinkle in 3 tablespoons of granulated sugar, 2 tablespoons of cornstarch, and 1 teaspoon of vanilla extract. Gently toss the mixture until the fruit is well-coated, allowing the sugar and cornstarch to meld with the natural juices. Set the fruit filling aside while you prepare the cobbler dough.

In a separate medium bowl, whisk together 1 ¼ cups of all-purpose flour, ⅓ cup of granulated sugar, 1 ½ teaspoons of baking powder, ½ teaspoon of cinnamon, and ¼ teaspoon of salt. Next, cut in 5 tablespoons of cold unsalted butter using a pastry cutter or your fingers until the mixture resembles small peas. Stir in ⅓ cup of buttermilk until just combined, ensuring not to overmix for a tender cobbler dough.

Lightly spray the bottom of your slow cooker with cooking spray to prevent sticking. Crumble the cobbler dough evenly over the bottom of the slow cooker. Carefully stir the prepared fruit mixture and spread it evenly over the dough, leaving behind any excess juices to avoid a soggy texture.

Cover the slow cooker with its lid and set it to cook on high for 2 ½ to 3 hours, or on low for 4 to 5 hours. You'll know the Crockpot Blueberry Peach Cobbler is ready when the filling is hot and bubbly, and the top has a cooked appearance. Check occasionally to monitor the cooking progress.

Once cooked, allow the cobbler to rest for 20 to 30 minutes to set before serving. Serve warm, and if desired, pair with a scoop of vanilla ice cream for an irresistible dessert experience.