Preheat the oven to 350 degrees F (175 degrees C). Grease a 9-inch springform pan.
Beat butter in a large bowl with an electric mixer until creamy. Add sugar and salt gradually and mix in well. Add eggs one at a time, beating well after each addition.
Combine flour and baking powder, sift, and quickly beat into the batter on low speed. Thin batter with a little milk if needed. Pour batter into the prepared springform pan. Arrange nectarines and blackberries on top in a decorative pattern.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 45 minutes. Dust with confectioners' sugar and cool on a wire rack for 5 minutes. Run a table knife around the edges to loosen. Invert carefully onto a serving plate or cooling rack. Let cool completely.
