Preheat the oven to 375F and set an oven rack in the middle.
Melt the butter in a 10-inch cast iron or nonstick baking dish. Swirl the butter to coat the sides of the pan. Scatter the blueberries in an even layer in the skillet or baking dish.
In a blender, combine the eggs, milk, cream, 1/2 cup sugar, flour and salt. Mix on medium speed until smooth and well combined. Pour batter over the berries.
Bake for 20 minutes, then remove the clafoutis from the oven and sprinkle with remaining 1 Tbsp of sugar. Place back in the oven and bake until set and golden brown, about 25 to 30 minutes more. Let cool on rack for 30 minutes, then dust with powdered sugar and serve.