All i can see is that textures are not loading.
Game loads the skyboxes for each map from client and all works fine for most people.
Im just not sure if it has something to do with this method, if its a roblox bug or the person has done something with their graphic settings.
It seems like when roblox had updated for them it may have failed to download some textures. Me and my friend had a problem like this in the past but our cursors were missing. We fixed it by reinstalling roblox.
Ok since people are still finding this post, im just gonna say that you experience black skybox when you delete textures. If atleast one part of the sky has a default texture, the whole skybox will turn black.