I was browsing through the .esm files and noticed that many dungeons had a "blackplane01" object. What are its uses?

I dont know if this is it's official use, but i use it to obscure things that i dont want to be seen yet or at all. Provided i can make it seem like there is nothing there instead of a black wall, so kinda only works somewhere where you would expect there to be black, like say at the end of a cave entrance where the geometry is being difficult.

i always use them for custom load doors that don't have a proper door well, so that when you creak the door open before travelling to a new cell, you dont see the stuff behind the door (which is sometimes a brick wall, which wuoldnt make any sense to the player)

They hide anything under them from the local map. If you have a secret half of your level, and you dont want the player to see it by looking at their map, put a black plane over it.

really? thats awesome i needed something that does exactly that!

I wondered what that was about.

What happens when they do discover the secret area, does it nullify the blackplane and the area is then visible on the map?

Hmmm, I don't remember. I think once you pass under it, it no longer 'counts' and the area under it is revealed, yes.

So, a little off-topic, but since Amethyst Deceived mentionned it, is there a proper "black" object to hide what's behind custom load doors ?

I use the black plane as well behind load doors. Also, I have a second underground section that opens up as my quest progresses. I have a black plane over the entrance to that tied to an enable parent that disables it when appropriate (and enables some other things at the same time). Works great.

Also, I use black planes to hide some odd bulges in my dungeon so the rooms look cleaner (some things I can't check "Hide from local map" on)

Wait, people actually use the local maps? The dungeons are so linear in Skyrim that I've not used them once...

Wow, did not know that, thanks! I was wondering why some places had them facing upwards but weren't even where the PC and NPC's can go...

Otherwise I use them to cover things up that I do not want to be seen and should remain dark, you just have to get the positioning correct. For example a well with a long shaft downwards, if you place one on a tile you can still see part of the tile when you look in the well. It would be good if there was also a round one though.

If this isn't the case, it could probably be scripted to do so fairly easily using a trigger volume and Disable().