If you want solid black textures on the walls and floors
1. get Firma
2. load it up
3. click create room
4. check the boxes "texture floor" and "texture wall"
5. make sure the box that says "make black transparent" is unchecked
6. hit load textures and one of the textures that comes with Firma is solid black I think....if not it should simple enough to create a solid black texture in paint, gimp, or any paint program
7. type in what you want your segment to be called, hit save and voila! black segment!
Find Firma here:
http://fps-files.com/index.php?site=files&file=138
Hope this helps! If you need anything else lemme know.
EDIT: this was done from memory so if some things aren't exactly true then sorry but it should be close enough.
It is common sense to take a method and try it. If it fails, admit it frankly and try another. But above all, try something. ~Franklin Delano Roosevelt