I'm having this same problem too (all all very very low settings) and I quess that it's simply reason of old/bad graphics card. (by using word old I don't mean necessarily much used card, but also many years ago released cards, out of date cards).
I'm having Nvidia GeForce 7300GT all drivers updated and I got aswell this black sky problem in some maps. But I'm not complaing since my card is far too bad for game like this, but coz I don' have money for new box, I gotta survive with this.
As there's been many issues wih graphics performances and bugs even for ultra computers, I think that it takes a lot of time for TW to get ALL graphics issues fixed. And we are just minority. But hopefully they note us some day, hopefully I get/have enough money for new box some day.