I haven't verified it in-game but for UW2 the black sword you find in the third level of Praecor Loth's tomb (the maze) is standard and has no special effects. However, the one that you find on the next level, on Lethe, appears that it may be enchanted to have Great Accuracy. You should be able to add that enchantment yourself with the VOY spell so it's not necessarily "special" in that regard. They don't have other magical effects as far as I know unlike some of the other special weapons.