Preheat your oven to 375°F (190°C). Lightly grease an 8-inch round baking dish or pie plate with butter.
Gently scatter fresh blackberries evenly across the bottom of the greased baking dish in a single layer.
In a medium mixing bowl, whisk together the flour, sugar, and salt.
In a separate bowl, whisk together the eggs, milk, melted butter, and vanilla extract until well combined and smooth.
Slowly pour the wet ingredients into the bowl with the dry ingredients. Whisk constantly until you have a smooth, thin batter.
Carefully pour the batter evenly over the blackberries in the baking dish, ensuring the berries are covered.
Place the baking dish in the preheated oven and bake for 40-45 minutes, or until the clafoutis is puffed up and golden brown around the edges and the center is just set.
Let the Blackberry Clafoutis cool in the dish for about 10-15 minutes before serving. Dust with powdered sugar if desired.