I intentionally induced a black horizon yesterday. When in spectator mode inside a mob's head I tried to teleport him 1000 feet up. Sometimes it works, sometimes it doesn't. So when it didn't work I tried it while out of the mob and he didn't get teleported but I did. So I teleported back down and tried it again but accidently used the -1000 code. Once again he didn't teleport (and neither did I since I'm anchored to him when in his head) but since the destination would've been in the void suddenly there was a black voidy sky (but with clouds still in it, although darkened). I jumped out of his head and it went away but I repeated my steps and got it to happen again. This time it was a black bar (middle 1/3rd of the sky). Repeated the experiment 4 or 5 times to be sure it was reliably repeatable. The only problem seems to be the randomness of if you get a black bar or the full black sky (couldn't figure out how to pick that). Note that when the teleport fails it returns a "entity not found" for the mob I'm in and returns a "player successfully teleported to <x y z>" for me. It's confusion over that might be why the sky changes to try to match what I would see at the destination.

BTW the code I was using was "/tp @e[r=5] ~0 ~1000 ~0" (without quotes), as in "teleport any entity within 5 meters of my position 1000 meters up from their current position". I count as an entity as well so I get teleported unless I'm inside the head of a mob that doesn't get teleported even though it should.