“Henceforth, Neltharion would be known as Deathwing. His treachery would curse his own black dragonflight to a life of fear and seclusion. In the coming ages, they would find themselves hunted to the brink of extinction by the other flights due to Deathwing’s unforgivable betrayal.”
This is right after the Demon Soul and Neltharion’s betrayal. It seems to implicate that the Black Dragonflight did not immediately succumb to the Old Gods as he did.
It still leaves the frustrating question of when they succumbed or how.
The Old Gods like to find a person’s weakness and exploit it. Even Neltharion thought the choices he was making were right and just. The only flaw was the increasing paranoia that the whispers influenced him with lead him to believe his allied were working against him.
So, imo, after the rest of the flight (those too proud/strong/moral to blindly follow him down the path of destruction) were ostracized by the other flights. There would have been fear and paranoia from the other flights and that would lead to some severe alienation.
From there…it’s easier to prey on the weakness of dragons who have no leader and no support system. They would likely be alone, or in groups with other black dragons.
Those alone would succumb to the whispers of the Old Gods who would prey on their solitude. Madness would find them in their solitude. The Old Gods would of course be the ones to encourage their feelings of despair until they returned to Deathwing because they could no longer handle the separation.
Those in groups would have a hard time picking up the pieces of their lost flight and rebuilding. It might take them longer, but after a certain amount of time they would begin to suffer together. I can see the Old Gods preying on their sense of betrayal and rallying them against the other flights. Once that seed of anger is planted, it would fester and these small packs would help it to grow by fueling and spreading this shared anger amongst each other.
At some point their shared anger would have them consider returning to Deathwings side since they now share the same goal and feelings towards the other flights. Should they return immediately or not, this train of consideration would make it easier for the Old Gods to stoke their anger and whisper how Deathwing had been right all along about Alexstrasza.
At least that’s my theory any who. I sort of see the Old Gods as being the worst mental illness you can possibly imagine because it’s alive and knows every single weakness and thought you have then it exploits it.
The sad part is that, by pushing them away and hunting them down, the other flights made the Old Gods job easier. If they had, instead, embraced their Dragon brethren, it would at least have saved many of them from following the path of their Patriarch.