Thinking about it, I think the reason there are more combat-y Naggaroth dragons and ones willing to fight on the Druchii side, versus the Ulthuan dragons being harder to rouse and harder to get to fight, is...
Is because Ulthuan dragons are like the equivalent of civil war veterans or WW1 and WW2 veterans. Storied and venerable and greatly respected people. People with their own opinions and thoughts and motivations. A
society of people, too.
While the Druchii dragons are more like... gang members on PCP or methamphetamine. It turns out it's a lot easier to drag somebody to the battlefield if you do so in chains. Even if controlling them is a bit trickier. And of course a dragon is a very dangerous being anyway, so it's always skilled and ferocious in a fight. Not a society of beings so much as just another resource for the war effort.
That's sort of the difference in what they are; ancient respected heroes and veterans and sages, versus child soldiers or gang members with a bunch of guns.
The Asur really love their dragons man. The benefit of being the dude who returned a tainted egg to the High Elves to be cleansed and brought into the world as a beautiful dragon, something that the elves are always lacking, far outweighs what would likely be either a mild physical boost and the destruction of our relationship with them, or if you mean keeping and raising it, then we have a vicious, feral, hateful little beast (since it's a damn Black Dragon) that'll be worse than Oskana ever was and will probably have to be put down in the end. Smartest option will always be let the Asur handle the dragons
Heh. Who do you think would be happier about hearing of a reclaimed Black Dragon? The Nagarythians or the Caledorans will be most pleased by this?
On the one hand; restored dragon, yay, Caledor happy!
On the flip side; 'Instructions unclear, instead of stealing Druchii child and raising it as a Nagarythean, accidentally stole Druchii dragon and raised it as an Ulthuani dragon.' What a flex. What a giga flex.
More seriously though, I think that while of course the Asur would prefer to get it and would be very happy with us for doing so... I think -- depending on circumstances, how things turn out, and whether we get a boon from the Asur or can ask a boon or etc -- if we asked them "Hey, can you help cleanse this dragon egg for us and help us raise it?" they might be willing to do so. i.e. That since they care about Ulthuani dragons (and what the Druchii do to dragons!), they might be willing to make sure we "Treat them right" and be willing to send somebody to help us raise it.
But it would probably require us using a boon, or asking for it, or etc.
Whereas if we handed it over to Ulthuan -- or to the Eonir -- we would be
getting more good will/money/support/whatever.
Having a compound of Asur helping to hatch and raise a dragon could be really cool though. Or maybe they'd move to the Arcane Pinnacle and do the raising there, since that's an already-secure position. But, having more Asur around, would be interesting and neat. More investment from them. More trade or connections? Or chance to talk to or befriend more Asur? And decades in the future, having a dragon willing to defend Ostland. And also possibly still serving as a connection to the Asur too -- i.e. if we have a dragon that they helped raise and taught us how to care for said dragon, and then if some Asur stuck around because they wanted to, then we'd have Asur around in Ostland, and we'd have an interesting connection to Ulthuan that way. That is, being able to open dialogue with Ulthuan at times if need be. Having some to connect over with the Asur. Useful in times of stress or trouble or uncertainty.
Alternatively, if we give it to the Eonir, well, dunno if they'd turn it into a Forest Dragon like the others or if they'd keep it a normal dragon, but. How to turn this into something that we would jointly be doing though? Unless we ask them to help us rear the dragon instead of the Ulthuan elves... but they aren't the Ulthuani so who knows if they'd be as knowledgeable... plus the Eonir came to help us here so there isn't exactly a big boon to ask of them so (unless something happens in the Beast-tide) hrmgh. Eh.
Alternatively, I wonder if it'd be possible for Eldyra to become a dragonrider? She isn't a mage or a Caledoran though, so... It'd be cool though. ... Not sure if she'd have
time for this though. In between being Tyrion's squire, then becoming a knight proper, then probably still being a companion and party member of Tyrion's.
All this is a bit too soon to know for sure though. Not only have we not dealt with the Beast-tide yet, we haven't even dealt with the Druchii yet. We haven't survived or thrived or won yet. Though I guess it's neat to speculate about.