Place the cake mix in a large bowl. Using a standard grater, grate the chilled butter into the cake mix and, using your clean hands, toss to coat the butter in the cake mix. Break up the lumps that form in the grated butter until the mixture is uniform and looks like very coarse sand. Scatter the butter and cake mix mixture on top of the blueberries, and stir gently to mix. Pour the chilled buttermilk evenly over the top.