Hm I do not know even if Hino can answer to that question hahaha
My understanding from what we know so far is that the BR obeys Kaname and that’s why he cannot get killed from those veins.
However as an anti-vampire weapon must have the ability to kill him I mean if Kaname let it.
Another thought that crossed my mind is that the BR obeys whoever is the strongest.
Because normally it should obey Zero i.e. its wielder from which it obtains blood … the veins are stemming from Zero’s body viz we can say that they are an extension of Zero’s body’s veins.
But every time that Kaname orders it the veins retreat. So could it be because the BR recognizes him as stronger?
Yet, should an anti-vampire weapon obey a PB over a hunter?
Another thing is why Kaname and some of the Kurans can wield anti-vampire weapons. Perhaps if Hino sheds some light over this issue we will be able to explain better the connection between Kaname and BR.
Even if the sword that Kaname wields weakening him –as Sara claims- still he is able to wield it something that none other PB can.
Another factor to this equation is Kaname’s words in the first arc http://manga.animea.net/vampire-knight-chapter-46-page-6.html
The BR recognized his voice and the taste of his blood as well, why?
Is it cuz Kaname was its first owner back in ancient times thus it was fed from his blood; or is it cuz of the HW’s soul/spirit that are transferred into the BR?
Then again if it is only the HW’s soul; how it can recognize Kaname’s blood taste? As far as we know Kaname only took HW’s blood but not vice-versa. So?
Another thingy that triggers my curiosity about BR is its name … how, when and why was named Bloody Rose? In ancients times it hadn’t a name so it must have took its name later and perhaps its name symbolizes something lol.
All these questions I think are related in a way or another with Kaname’s relation with BR.
The simpler explanation would be that Kaname is so powerful that nothing can kill him except if he allows it but personally I want something more than this lol.
We have discussed the anti-vampire weapons and Kaname’s relation/contribution in their creation lengthily in this thread for anyone that would be interested > https://vampireknight.all-up.com/t140-anti-vampire-weapons
I still think that some of those theories might be valid and in my logic are explaining a lot.