In Skyrim, each exterior cell is made up of four quadrants and each quadrant can have no more than six unique texture types painted in it. If you try to add a seventh, the texture will simply paint as a matte black color; it will turn black in game as well. Not to fear, this isn’t like the black texture bug of Oblivion infamy, this black paint can be undone, or painted over, just like every other landscape texture. If this happens you’ll need to consolidate your textures.