http://forums.nexusmods.com/index.php?/topic/1163434-enboost-lighting-and-black-horizon-issue/ In case you are using ENBoost then visit the thread in the link above and perform the changes outlined in it. This made my black horizon go away.
http://forums.nexusmods.com/index.php?/topic/1163434-enboost-lighting-and-black-horizon-issue/ FishBiter, SilverWolfMNG The above thread contains helpful information. Please report back if the steps in it eliminate the black horizon issue for you.