Mix the plums, blackberries, granulated sugar, salt, and cinnamon together in a medium sauce pot and cook over medium heat until the fruit is tender and warm and the juices have released.
Strain the fruit and return the juices to the pot. Cook the juices over medium heat until they're reduced and slightly thickened. Whisk a small amount of the reduced juices into the AP flour to create a slurry. Pour the slurry back into the juices and whisk to combine. Stir the fruit back into the juices and allow to cool to room temperature. While you wait, put your oven rack in the middle and place a baking tray lined with foil on the rack. Preheat the oven to 350 degrees Fahrenheit.
Stir together the oats, flour, brown sugar, baking powder, kosher salt, and cinnamon. Melt the butter and pour over the mixture. Stir it together until the dry bits are saturated and begin to clump together.
Fit a 9 inch pie plate with your pie crust. Pour the fruit mixture into the crust and smooth it out. Using your hands, sprinkle the crumble topping evenly over the pie. Bake on the foil lined tray (to catch any juices) for 55-65 mins until the crust is golden brown and filling is bubbly.
Remove from the oven and cool completely before serving. Enjoy with a scoop of vanilla ice cream, drizzle of cream, or on its own!