That's a good question, I was assuming that only the Meisters were able to see them, but maybe the weapons too, and it goes without saying that also the Shinigamis should. But so far, all seems to point to only the Meisters, since they have been defaulting to them so far... I think Tsubaki was wondering about Black Star... if weapons would be able to see them, I would think that Tsubaki would be capable. With that said, I don't think this anime would have a strict rule about it, since it's whacky enough to bullshit it's way through any given situation.