black threads have only been seen under very specific circumstances in the books. Even rand doesn't see them normally. He sees them in wherever it was he fights Ishy the at the end of EotW(did we ever determine if that was T'A'R', or what?), and in the skimming platform place. Neither of which have rules which function exactly like they do normally. So we can probably assume that the black threads have always been there...humans just normally can't see them, except in certain places.

Now that I think about it, the book specifically says that Rand can't see Asmo's black thread once he is out of the skimming platform place. He imagines where it would be based on where he saw it before and cuts it.

Also, I sent you a noteboard note. click up on the noteboard link at the top of the page.