Well with FJ, they didn't have much of a choice. If they put bricks or rockwork on it, it would extend the "mountain range" out too far and throw off the scale of Hogwarts even worse. I think they did they only thing they could - leave it bare and hope it blends in with the sky. While it's certainly worse than a properly located building (from certain angles the visible show building really kills the scale of the castle), they probably made the best artistic choice by leaving it blank. Further rockwork might only call more attention to the unsightly shape/scale.