Why does Brynden River Bloodraven have firstmen abilities?

[JonBranRickon]

17 hours ago, Duranaparthur said:

Prophetic vision, and a great deal of the other such classical magic capabilities displayed by the Targaryens, may not be limited to their genetics, but instead merely more present in that bloodline. Individuals like Maggy the Frog or the Ghost of High Heart display capabilities very similar to the dragon dreams, while Nettles seems to be a likely candidate for a "dragonseed" who's not actually "Dragon"-seed. There's clearly something otherwordly about genetics in Westeros, but it sometimes seems like the Targaryens' more present display of such abilities is down more to a high profile and greater opportunity to display such abilities: the Ghost of High Heart clearly delivers consistently accurate if metaphorical dreams, but she's an ancient lady stranded in the wilderness, not a powerful ruler sitting in a keep with access to ancient prophecies to bolster and hone her abilities. And there's some question about whether "diluted" blood dilutes magic powers or not: the Starks and Baratheons are two examples of high houses with peculiar re-occurences of common powerful traits, which may be tied into their origins (possible Children blood in the Starks, Storm God blood and Dragon blood in the Baratheons) and the modern Targaryens went through what would theoretically be a magic-destroying series of marriages outside the family, particularly Maekar (married a Dayne) and Aegon V's (Blackwood!) line, which Dany is directly descended from, yet are a totally Targaryens. 

Greenseeing seems to be the only magical power that does have some kind of ethnic bond; the only people using such abilities have the blood of First Men, but also the culture of First Men. And the latter factor may have a great deal more to do with expressing that power than the blood. We've got no evidence for First Blood "purity" playing a part in these abilities: the Blackwoods especially have a varied genealogical history, considering they're surrounded by "Andal" Houses, and considering how the Starks and Mormonts both have either evidence or family legends of warging, but lack the wildling subculture that created Varamir, having a knowledge base over how to actually use those abilities may be the real key.

Again, Dragon's blood probably increases your chances, but it doesn't guarantee those abilities. Heck, considering how often the lines of Bracken and Blackwood have crossed over, Bittersteel's failure to display such abilites (as of yet) may show even more how the eugenics of fantasy may not really work. Its entirely possible that Bloodraven took his rare gift and honed it becausehe had access to First Men traditions and Targayen patronage to practice his powers, just as its possible that Targaryens born with gifts may have let them atrophy or discounte dthem because their maester, the pseudo-scientists occasionally equipped with an anti-innovation bent and quite probably anti-magic bias, discouraged it... 

House Braken broke from worshiping the old gods; it seems to me that greenseeing is distinctly a gift for those followers.    Perhaps if Bittersteel worshiped under a weirwood he to might have the power. 

[Duranaparthur]

Which again plays into the cultural side of ethnicity with the green seer powers; the blood may not matter if honoring a heart tree is the key.

[John Suburbs]

I don't think Blackwoods count as "ice". Only Starks are ice. They got it from the Night's King, who fathered children on (most likely) an Other. If the NK was killed before he could sacrifice his last son to the Others, then that child would become the new Lord of Winterfell and the entire Stark line would be infused with ice blood.

Moreover, the union of ice blood and fire blood is a violation of the truce that ended the Long Night, which is why the Pact of I&F was never consummated, probably at the request of the Green Men. Then Rhaegar and Lyanna hooked up to produce Jon, and that's what got the Others on the warpath after all these years.