In a 10-inch enamel-coated cast-iron skillet, melt ½ cup butter over medium heat. Stir in brown sugar; bring to a boil. Stir in 3 tablespoons bourbon; cook for 1 minute. Remove from heat. Place peaches in skillet, and sprinkle with blueberries.
In a large bowl, beat granulated sugar and remaining ½ cup butter with a mixer at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition.
In a medium bowl, whisk together flour, baking powder, and salt. Gradually add flour mixture to butter mixture alternately with milk, beginning and ending with flour mixture, beating just until combined after each addition. Stir in remaining 1 teaspoon bourbon. Spoon batter over fruit, smoothing top with a spatula.
Bake until a wooden pick inserted in center comes out clean, 40 to 45 minutes. Let cool in pan for 10 minutes. Invert cake onto a serving plate, and serve warm.
Recipe by Southern Cast Iron at https://southerncastiron.com/peach-bluberry-upside-down-cake/