Adjust oven rack to middle position. Preheat oven to 350°F. Cut 4 tablespoons of butter into pieces and place into a 9 x 13-inch baking dish. Melt butter in oven, about 8 minutes.

In small bowl, combine 1/4 cup granulated sugar and lemon zest. Rub together. In large bowl, combine blueberries and 1 tablespoon lemon-sugar. Using potato masher or large fork, gently mash the blueberries until most of the berries are broken. It's fine if a few of the berries remain whole. Set aside.

In medium bowl, whisk together white rice flour, potato starch, sweet rice flour, baking powder, salt, and xanthan gum. Melt remaining butter and allow to cool slightly.

Remove baking dish from oven and place on wire rack. Carefully rotate the pan so the melted butter evenly covers the bottom of the pan. Add the 8 tablespoons melted butter and milk to the dry ingredients. Whisk until the batter is smooth. Pour batter into the baking pan. (The melted butter in the pan might run to the edges of the pan and up the sides of the batter. This is fine.) Dollop the mashed blueberries over the batter. The berries might sink below the batter. Sprinkle the top of the cobbler with remaining lemon-sugar.

Bake until golden brown and set, about 50 minutes. Remove cobbler from oven and place on wire rack. Cool at least 30 minutes before serving. Serve warm or at room temperature.