Place butter in a cast iron skillet. Place skillet in the preheating oven until butter melts, about 5 minutes.
Combine eggs and milk in a bowl; whisk until combined. Whisk in tapioca flour, coconut flour, lemon juice, lemon zest, and sea salt until combined. Fold blueberries into the batter.
Remove skillet from oven using a pot holder or kitchen towel. Spread melted butter with a pastry brush to coat skillet evenly; pour in batter.
Bake in the preheated oven until middle is firm and top is golden brown, 18 to 20 minutes. Slice and drizzle with maple syrup.
