I was trying to level blacksmithing when I noticed this:
I looked up the key and found that it should only open boxes up to 125, so I tried opening something higher than that (all I could find was a 225 lockbox). Amazingly enough:
And then it opened. The game says it's an obsidium skeleton key, and thus can open everything.
Make some golden skeleton keys and have fun opening stuff while it lasts!