Adjust oven rack to middle position and heat oven to 400 degrees. Brush bottom and sides of 8-inch square baking pan with melted butter.
Whisk flour, sugar, baking powder, baking soda, and salt together in a large bowl. Add chilled butter to flour mixture and smash butter between your fingertips into flat, irregular pieces. Add blueberries and toss with the flour mixture. Gently stir in buttermilk until no dry pockets of flour remain.
Using a rubber spatula, transfer dough to the prepared pan and spread into an even layer and into corners of the pan. Using a bench scraper sprayed with vegetable oil spray, cut the dough into 9 equal squares (2 cuts by 2 cuts), but do not separate. Bake until browned on top and a paring knife inserted into the center biscuit comes out clean, 40 to 45 minutes.
Meanwhile, combine butter, honey, and salt in a small bowl and microwave until the butter is melted, about 30 seconds. Stir to combine; set aside.
Remove the pan from the oven and let the biscuits cool in the pan for 5 minutes. Turn biscuits out onto a baking sheet, then reinvert biscuits onto a wire rack. Brush tops of biscuits with honey butter (use all of it). Let cool for 10 minutes. Using a serrated knife, cut biscuits along scored marks and serve warm.
