Ok, here is my theory:
Is that the black city is Arlathan itself. There is an tale that Andruil went to the void, the place were the forgotten ones dwell, and found a corruption that gave her great power, and she made an armor made of that corruption, and in her wars for power, Andruil spread great misery and plagues. Andruil brought the taint, the blight. And in that war Arlathan was corrupted. When Solas created the veil the trap the elven gods, he also use it as an way to keep the corrupted city away, hence, the black city was born.
Also, the legend says the dread wolf trapped the forgotten ones in the abyss. I believe it means the old gods in the deep roads.
I'm very curious to see what The Void is, and how Andruil got there. There are apparently creatures there, since she hunted them. It must be where the blight originated. For some reason I forgot she spread plagues, so that fits too. Ah, the abyss is the deep roads, or beyond. That makes sense!
There's nothing in the lore about Arlathan being corrupted and we know it still existed in Thedas at the time of the Imperium, they laid siege to it and sunk it beneath the earth when that proved two costly. Plus, the city is known to have appeared Golden prior to the arrival of the Magisters.
As previously said, that's not true, it was destroyed from within. My personal theory is that it is in the fade, forced there when the veil formed like the Great Library, since they were both magically-powered/held together. I also think it's the Black city, but it needn't be. That would also explain why people thought it must have sunken into the ground, since it disappeared so seamlessly.
As for the Golden City, it's still possible it's Arlathan, and the blight was contained within, then when the magisters breached it, they released the corruption and it turned black. Then they brought it with them to the material world. I think there was still blight in the material world at that point brought by Andruil, but not much, and only in the deep. When the magisters brought it back, it allowed it to spread faster and ended up causing blights.