Mmm, true, but I'd discounted the Cult of the Great Dragon Hunter because they're inherently linked to Verjigorm (the Great Dragon Hunter hisself!), and the Horrors aren't even hinted at as a threat in any material published since Dunkelzahn offed himself - well, that I've got access to anyway. If they were involved, I'd expect to hear more noises about Cthulhuesque cults trying to bridge the gap, as well as that guy from Harlequin2/Threats2, what was his name? Something Black?
Whereas the BL/Denairastas connection is hinted at multiple times, and they fit the bill as enemies of dragons as well as anyone.
Ancient Enemies of Dragons that could be stirring:
1) Therans
2) Cult of the Great Dragon Hunter
3) their children/Dragonblooded elves
4) Denairastas
Therans are another possibility but as the Atlantean Foundation seems almost more academic than political, and the other organizations who look for Theran artifacts seem bent on using them for their own power rather than resurrecting the 'glory days' I discounted them as well.
And whether the rules for sacrifice metamagic allow willing or unwilling donors, from the start it's been restricted to NPCs because of the moral implications, and even in the 5e book it says, "Be cautious when allowing PC blood mages" to GMs. Damfool idea to even hint at allowing it to PCs; they sure don't talk about toxic or insect shaman PCs...
Whereas in Earthdawn, it's available from the start. Sacrifice some of your damage capacity and spend some silver, and you too can have on-call fiery wings which let you fly to the floating castles of Thera! Or if you need to, take some damage to bump up your chances of performing that dangerous stunt! There are no moral implications, and I can't recall reading much of anything about unwilling donors in the books. Maybe my memory has just failed in my old age, (and I don't have the books close to hand) but even the Therans didn't use unwilling blood magic sacrifices... even the kaer magic didn't require it, and if anything did it would be that. The only ones who used unwilling donors were Horrors - and I always got the feeling they did it more for the horror it would inspire than because they had to...
And, er, the 'murderous hate-boner' line is one I use when I'm trying to actually damp down the rhetoric of an argument; I always laugh so hard when using it that I... well, I'm sorry for using it.