Preheat oven to 425 degrees F (220 degrees C). Generously butter a 2 1/2-quart baking dish.
Pour blueberries into prepared baking dish.
Blend milk, sugar, flour, eggs, vanilla extract, and salt in a blender until batter is smooth. Pour batter over blueberries and gently shake to remove any air bubbles.
Bake in the preheated oven until puffed and center is set, 25 to 30 minutes. Cool until clafoutis deflates and is just warm.