Place an oven rack in the middle position and preheat the oven to 450F. Set a 9- or 10-inch cast iron skillet over low heat on the stovetop and keep it there while you make the batter.
Melt 2 tablespoons of the butter in a small skillet, then transfer to a blender. Add the milk, flour, eggs, granulated sugar, vanilla, salt, and lemon zest and blend until smooth (do not skip the blender or the batter will be lumpy).
Put the remaining 2 tablespoons of butter into the hot cast iron skillet and turn the heat up to medium-high. Let the butter melt, add the blueberries, and give the skillet a shake. Immediately pour the batter over the blueberries and transfer the skillet to the oven.
Bake until the pancake is puffed and golden, about 15 minutes.
Dust with confectioners sugar and cut and serve immediately with maple syrup.
