Use fresh blueberries and peaches that aren't super ripe for this cobbler recipe so they'll hold their shape when cooked. The baking dish will be brimming with fruit and topping, so it's a good idea to place it on a foil-lined baking sheet before putting it in the oven.
Place peaches in a large bowl. Drizzle with juice; toss. Add 3/4 cup granulated sugar, 1/8 teaspoon salt, and 2 tablespoons flour to peach mixture; toss to combine. Arrange peach mixture evenly in a 13 x 9-inch glass or ceramic baking dish coated with cooking spray.
Weigh or lightly spoon 75 ounces flour (about 1 1/2 cups) into dry measuring cups; level with a knife. Combine 75 ounces flour, remaining 1/4 teaspoon salt, and baking powder in a bowl, stirring well with a whisk. Place the remaining 1/4 cup granulated sugar and butter in a medium bowl, and beat with a mixer at medium speed until light and fluffy (about 2 minutes). Add eggs, 1 at a time, beating well after each addition. Stir in vanilla extract. Add flour mixture and buttermilk alternately to butter mixture, beginning and ending with the flour mixture, beating just until combined. Stir in blueberries.
Spread batter evenly over peach mixture; sprinkle with turbinado sugar. Place baking dish on a foil-lined baking sheet. Bake at 375° for 1 hour or until topping is golden and filling is bubbly.