To make the streusel:
In a medium bowl, use a fork to stir together the flour, brown sugar, salt and cinnamon. Add the butter and, using your fingertips, quickly work it into the dry ingredients until pea-size crumbs form; set aside in the refrigerator.
To make the blueberry nectarine buckle:
In a medium bowl, whisk together the flour and baking powder; set aside.
Set the oven rack in the middle position. Preheat the oven to 350 degrees F. Butter a 9-by-13-inch baking dish; set aside. Line a baking sheet with a nonstick silicone baking mat; set aside.
In the bowl of a standing mixer fitted with the paddle attachment, beat the butter, sugar and salt on medium-high speed until light and fluffy, 2 to 3 minutes. Beat in the eggs one at a time, and then the vanilla. On low speed, beat in the dry ingredients just until absorbed. Fold in the fruit. Transfer to the prepared baking dish. Sprinkle the streusel over the top.
Set the baking dish on the prepared baking sheet. Bake, rotating the sheet about two-thirds of the way through the cooking time, until a cake tester inserted into the thickest part of the buckle comes out clean and the topping is nicely browned and crisp, 30 to 35 minutes. Transfer to a wire rack to cool for 10 minutes. Dust with confectioners’ sugar and cut into squares.