I tried that once in an alternate reality and it was a mess, I think that was how we found out Treatyseed was affected by the blight in the first place. Not only will reclaiming cause a massacre under the dwarves living in Treatyseed, but for some reason dozens of dwarves migrated into the fortress from other sites within the few days I played it, most of them were torn to shreds in seconds. I don't know how a party of seven inexperienced dwarves could have any hope of fixing that, without making everything worse.
CHALLENGE ACCEPTED!
...Ahem. If no one else tries by the time of my turn (and if the save doesn't turn into a raging dumpster fire by that time), I'll give it a shot.
But seriously, leaving it as it is is worse. A lot of the murdering is happening in the underground maze of that (by now) cursed fortress. Every new adventurer/possessed creature just loading the site is going to start another round of killing, possibly increasing the skills of blighted present super-dwarves who get better with each killing.
Now that I think about it, I'll have to probably leave everyone on the surface and seal off everything even after I clean everything up, because there'll always be the chance of someone blighted being called to the capital since it's not just the capital, but Evost Dèg, a major religion's Holy City.
You might have the best chance if you reclaim the site using the other dwarven civ, Nomal Alis.
Also immediately destroy the tavern, raze the temple to the ground and remove any other room and guild hall assignments that may exist.
I have some hope that changing ownership to Nomal Alis will stop the site from attracting Adilatir nobility and citizens, it should not have the same attractiveness to Nomal Alis.
I hope Adilatir will not see that as a declaration of war though..