The vertical side of a cobblestone block, y~33, viewed from a certain range of angle/distance, lights up as blue sky. Effect stops when block is removed. Effect reappears when replaced by granite, dirt, .. and maybe other materials. See enclosed mp4.
Maybe relating to this issue, while looking diagonal upwards in an underground gorge, I noticed blue sky silhouettes of grass, while the top of the gorge was under the surface. Upon excavating a ledge surrounding the top, inspecting all nooks and crannies, also from the surface, I did not find a hole in the top of the gorge. The effect was gone when I rearranged some blocks at the top of the gorge.