http://royals.ms/forum/threads/blackboard-and-nx-effect-glitch.5343/ It is still not working . Still doesnt work. Only people that was in the map can see the nx effect but not other pple. Uploaded with screenshot. I actually not sure if it was even fixed before even though the previous thread said so.
This bug will not be fixed in this source, all focus are on the new source where everything should and will eventually work.
On the beta I've been able to change maps with a chalkboard open and people could still see it, so it's on the list of things that will be fixed. In the thread you linked in your post, GI states that it was not actually fixed.