Preheat oven to 350 degrees Fahrenheit. Spray a 13x9-inch glass baking dish with cooking spray; set aside.

In a large bowl, add the peaches, blackberries, ½ cup granulated sugar, and cornstarch. Toss gently to combine; there will still be sugar/cornstarch that won't stick to the fruit and that's okay. Pour fruit into the prepared baking dish and sprinkle any extra sugar/cornstarch mixture over the top.

In a separate large bowl, add the butter, brown sugar, oats, flour, cinnamon, and optional salt. Using a pastry blender, two forks, or your hands, cut the butter into the dry ingredients until small pea-sized clumps form and some sandy bits form.

Sprinkle the crumble mixture over the fruit and bake for 45 to 48 minutes or until the fruit is bubbling around the edges and center and the crisp on top has browned. Serve warm with vanilla bean ice cream if desired.