Valyrian/Dragonlord blood is needed to tame/subdue dragons

[Mithras]

If they had a political reason to keep Aemon out of the inner circle in the Citadel it makes sense to assume that Targaryen magical blood had something to do with it, as any other political reason makes no sense considering that Aemon could have been an asset in pretty much every mundane political agenda.

They might have every reason to keep the Targaryens out of the hidden vaults containing dangerous books. But that is not about dragonriding.

[Consigliere]

I don't see any reason why other magically talented individuals can't be Greenseers. There's no evidence Bloodraven's magic came from his mother rather than he simply learned the magic of the North.

It is highly unlikely that BR simply "learned the magic of the north". His blood is what made him a greenseer and in this series, blood is important. The fact is the only wargs/skinchangers/greenseers we have encountered happen to have first man blood. However it is the "right drop" that seperates wargs/skinchangers from the rest of the population and greenseers from wargs/skinchangers.

[Lord Varys]

If you read the text, Marwyn says the Citadel cannot trust Aemon (or Marwyn) because of his blood. This has nothing to do with any books the Citadel may or may not have. Tyrion knows about the 'Blood and Fire' volume, suggesting that Aemon and others did to - if the Iron Throne wanted to consult any books the Citadel keeps under close guard all the king had to do is to demand that the Citadel hand them over to him, and he would get them. Hell, even Aemon would have been allowed to read it had he, a royal prince, insisted that he had to to continue his research.

The quote suggests that a Targaryen maester may have other goals and interests as a the common maester due to the specialness of his blood, and may thus not be all that interested in the common anti-magic agenda of the Citadel. And the important thing is that Marwyn actually believes there is a difference between people with Targaryen blood and 'normal people', and he also believes that the Citadel as a whole believes in that distinction.

Thinking about that: How likely is it that Marwyn also has Targaryen blood? He could be descended from one of Rhaena's daughters or be a descendant of Elaena either through the Plumm or the Penrose line. I guess the Rhaena thing is more likely, as Marwyn most likely has close ties to the Hightowers (if he is not Lord Leyton himself in disguise, as some people have suggested), or else he would not have been allowed to continue his research, I assume. Not if what he tells about the gray sheep is true.

On Bloodraven:

Nothing suggests that he was a very powerful skinchanger/greenseer, as it seems that he is not yet aware of his skinchanging ability in TSS/TMK - he is not accompanied by animals, nor does it seem that he is using animals as spies when he investigates Whitewalls. That's no conclusive evidence, as Bloodraven would have to be very cautious to keep his skinchanging abilities a secret, but if he had to do this, he could not really hone his abilities until her went to the Wall. I'd not be surprised if Bloodraven only consciously began to train to become a skinchanger after he had joined the NW - wildling skinchangers he interacted would have recognized him for what he was.

[barristan-b]

Blood from the male line can tame them. Blood from the female targs can hatch them.

[Damp Hair - The Prophet]

I think not.

The dragon horn, “Here the horn is named. 'I am Dragonbinder'" will work. I think Victarion &/or Euron will possess one for a time.

I think Tyrion 's saddle design skills and knowledge of dragonkind are not for nothing.

I think some of (1-3) Dany's three will die and some other as yet unknown dragons MAY appear.

I think the recruitment of dragon riders scenes in TPATQ short story is the best evidence.

[Sanset]

It seems that Targaryen Dragonblood is needed to ride a dragon as blood of the First Men is needed for skingchanging. Or Starkblood to do something "magic" in Winterfell, after all, Ned told Cat, "He is my blood, and that is all you need to know. And now I will learn where you heard that name, my lady." (that name: Ashara Dayne)
Nevertheless, it seems that most of the people that fights against this idea, it's mainly because they don't like it, which is very understandable. I don't like the idea either because this dragonblood magic seems more a curse than a blessing. And I really like Dany and Jon, so for me this dragonbond is more a bad omen than a super-cool magical awesomeness. I mean, it looks more like the bitter part of the bittersweet end or cliffhanger that was promised.

I've been looking at the Targaryan line. The house was slightly weakening because of outmarriages but those partners were not of houses aligned with any kind of element antithetical to Fire until Alicent Hightower enters the picture. The Hightowers are so closely aligned with water that they're even said to perhaps predate the First Men and possibly worshipped the the same God as the Ironborn. The Martells come a little later into the picture. They've got water alignment too from the Rhoynar. If earth and air are components of fire, then marrying into those houses might mix up the Targaryan genes but marrying into water houses, particularly High tower, could quite reasonably start 'quenching' the house of its fire affinities over the course of time.

I think is very interesting what you're pointing out. The relationship between Fire and Water works very well while none of them try to overcome the other. If they work in harmony it could be a very delicious and living team. Like Potatoes and Paradise. :)

If too much water is thrown over the fires, this turns off. And without enough fire the water turns into ice, so it doesn't flow anymore. If the fire is too strong for too long, the waters turns into steam and it's lost in the air. Otto and Alicent were too much water on the weak and unstable fire. They might have done it on purpose. I don't know, but it's been suggested in a very well thought theory posted in these boards.

ETA: Finally... I found the link to the theory: "The Gods Eye Conspiracy".

[The Vulture King]

On Bloodraven:

Nothing suggests that he was a very powerful skinchanger/greenseer, as it seems that he is not yet aware of his skinchanging ability in TSS/TMK - he is not accompanied by animals, nor does it seem that he is using animals as spies when he investigates Whitewalls. That's no conclusive evidence, as Bloodraven would have to be very cautious to keep his skinchanging abilities a secret, but if he had to do this, he could not really hone his abilities until her went to the Wall. I'd not be surprised if Bloodraven only consciously began to train to become a skinchanger after he had joined the NW - wildling skinchangers he interacted would have recognized him for what he was.

Pretty much agree with everything in your post about Tar blood being different and I can even see Marwyn having some but I do not agree with you on Bloodraven.

I think it is pretty clear that BR has figured out his skinchanging abilities by the TSS/TMK. My personal belief is that he was so powerful that he actually warged the man/archer that killed Fireball.

Lets also not forget that BR was also able to glamour by this point. Very powerful magic that if he could master he must have known he had other "abilities"

[Lord Varys]

TVK,

if the Theon chapter from TWoW and the second half of ADwD in general is an indication, then Bran is much more powerful a skinchanger/greenseer than Bloodraven ever was, suggesting that Bloodraven himself may not have been all that special in that field. Skinchanging humans seems to be a very difficult thing, and I'm pretty sure if Bloodraven had ever tried to do such a thing (and failed, which he would the first time) his own family would have killed him. It is much more likely that Bloodraven himself killed Fireball, disguised as some common man. The whole archery thing suggests as much (it could also have been one of his Raven's Teeth).

I don't say it is completely impossible/unlikely that Bloodraven had in TSS/TMK no idea what he could potentially do/become, I merely say that I don't think he was a powerful skinchanger by then. Mostly because his environment would have prevented him from studying and honing his inherent magical skills (unless we assume that there were Blackwoods who were skinchangers, too, at that time).

I guess he, Aerys I, Shiera, and the Grand Maester during Aerys' reign continued their magical research in their spare time, and possibly helped each other (and thus also Bloodraven) along, but I'm not sure whether you can become a powerful skinchanger without being guided by one of your own.