Preheat the oven to 425ºF with the cast iron skillet inside, so it can preheat with the oven.
Zest the lemon (you’ll need about 1 tsp), then squeeze about 1 Tbsp of juice.
In a large bowl, stir together the cornmeal, flour, sugar, baking powder, salt, and the lemon zest. Use your hands to massage the lemon zest into the dry mixture a bit to help release the flavorful oils.
In a separate bowl, whisk together the milk, oil, egg, and 1 Tbsp lemon juice. Pour the whisked wet ingredients into the bowl with the dry ingredients. Stir just until the dry mixture is completely moistened. A few lumps are okay.
Carefully remove the hot skillet from the oven. Add 1 Tbsp butter to the hot skillet and use a brush to spread the melted butter up the sides of the skillet.
Pour the prepared cornbread batter into the hot skillet. Sprinkle the frozen blueberries over top (no need to thaw), then push them down into the batter with your fingers.
Carefully return the skillet to the hot oven. Bake the cornbread for 20-25 minutes, or until the top and sides are deep golden brown and a toothpick inserted into the center comes out clean. Cool slightly, then slice into 8 pieces and serve.