Why can't a siphon absorb blood from a Doppelganger?
Are you referring to when Kai tried (and failed) to siphon magic from a human/cured (formerly vampire) Stefan?
Yes, It's possible?
No*. Stefan was human and could not be siphoned from.
*This could likely be a blooper/plothole because despite being human, doppelgängers are mystically recurring supernatural phenomena and their blood is used for potent spells. Furthermore, the cure is also of supernatural origin (like vampire blood itself) and if removed the former vampire ages.
Beyond Stefan's statement of "I'm human" it's never explained why Kai couldn't siphon from him as the last living [human] doppelgänger.
This is also questionable he should have been able to absorb the healing also after all has magic healing, I think it must be a defense of nature and blood Doppelganger has a lot of magic a witch diverting some blood would have enough power to destroy thousands of other witches with blood Doppelganger.
Technically a siphoner is supposed to be able to drain Doppelganger blood of it’s magic. Simply by touching the Doppelganger. But I also think that the reason why Kai couldn’t siphon Stefan was because he wasn’t directly touching Stefan’s blood. Since the magic is inside the blood, touching the blood directly should allow a siphoner siphon it’s magic.
I thought doppelgänger blood wasn’t spirit magic?
Doppelgangers have always been the most confusing of the show(if you ask me). Bennett might just be right. Then again Kai probably couldn't do to him trying to absorb the wrong thing(stefans vampirism which was no longer there). As there has been times when it seems Siphoners can choose specifically what they are siphoning. Like with Oscar siphoning just the visions from Bonnie or Val just siphoning wolf venom from Damon. 😎
^^Siphoners don't have to touch the blood directly. This has been shown, i.e. Kai siphoning the vampire blood/magic from Liz and the example Sean gave with Oscar.
^They were created by Nature and given what's known, isn't result of spirit magic, or any other stated magic (i.e. from a witch). Just that they are as a result of Nature.
Specificity could be it, as there's been a few examples of where it's occurred but never been elaborated on. Doppelgängers, by nature are supernatural - enough so to go to the Other Side. It really just seems like a plot hole, otherwise there'd need to be a convoluted reason as to why they can't.
What do you think?