Black weapon is the joint name of weapon that has no element status. Due to the damage compute mode of diablo iii and some items’ element damage stack, black weapon can get high damage. This bug was found at 2012 but community manager said the change would have a great influence on a number of weapon, so nothing was done to solve the problem. This time before the release of loot 2.0, a player asked this question again and luckily he got answer from game designer Wyatt Cheng.
The black damage problem would be fixed in loot 2.0. Diablo 3 console version adding % element damage is just like this. More details now are under discussion, of course.