Preheat the oven to 350°F. Lightly butter a 9-inch round baking pan or skillet. Pour the melted butter into the prepared pan, add the brown sugar, and stir together.
Arrange the peach slices and blueberries in a single layer over the brown sugar mixture.
Sift together the cake flour, baking powder, and baking soda. In a large bowl, cream the butter and granulated sugar until light and fluffy. Beat in the eggs and vanilla, then mix in the sour cream. Gradually add the flour mixture and mix on low speed until just combined.
Pour the batter over the fruit and smooth the top with a spatula. Bake for about 40 minutes, or until a toothpick inserted in the center comes out clean.
Cool the cake in the pan on a wire rack for 10 minutes. Carefully invert onto a serving plate and spoon any remaining fruit juices over the top.
Serve with whipped cream or vanilla ice cream if desired.
Notes
Adapted from Virginia Willis' Peach Upside-Down Cake. Serve warm or at room temperature with whipped cream or vanilla ice cream.