White texture is black
It may have been an overlapping wall? Some brush glitched out majorly.
Questions abound: What is the name of the texture material you were using? Were you using the same material when you finally got it working correctly? Was it white in the material selector, but black once placed on the wall in Hammer? Or did it display as white in Hammer, and was only black once compiled in-game?
I know I have seen some textures in the material selection window that have looked really different once placed. Some off-textures are just slightly different like metal/metalgrate018b, which looks like the other grates in the selector but has a circle in the middle once placed. Other off-textures, however, are quite significantly different like nature/blendwhitefloor_dirt01, which looks dusty but definitely like a white panel in the selector but is dirt floor and clumps of moss once placed.
If it displayed as white in Hammer, but was only black once compiled in-game, then that is really bizarre.