Adjust oven rack to upper-middle position and heat oven to 350 degrees. Place 4 tablespoons cut-up butter in 13 by 9-inch baking dish and transfer to oven. Heat until butter is melted, 8 to 10 minutes.
Meanwhile, pulse ¼ cup sugar and lemon zest in food processor until combined, about 5 pulses; set aside.
Using potato masher, mash blueberries and 1 tablespoon lemon sugar together in bowl until berries are coarsely mashed.
Combine flour, remaining 1¼ cups sugar, baking powder, and salt in large bowl. Whisk in milk and 8 tablespoons melted, cooled butter until smooth. Remove baking dish from oven, transfer to wire rack, and pour batter into prepared pan.
Dollop mashed blueberry mixture evenly over batter, sprinkle with remaining lemon sugar, and bake until golden brown and edges are crisp, 45 to 50 minutes, rotating pan halfway through baking.
Let cobbler cool on wire rack for 30 minutes. Serve warm.