@Poledo: Interesting theory. I think the Black City is Arlathan, black due to the fact that it's empty etc. Destroyed by in-fighting by it's own people and consumed by the fade for safe keeping. I have a heap of theories on the world of Thedas starting from DA:O going all the way through DA:I and beyond. I'll just have to wait and see if I'm at all right.
I think it's Arlathan as well, but black because it's tainted. It's the blackened heart of Arlathan.
I believe the fighting was due to corruption by the taint (we know it sent Andruil into madness), or it was used as a weapon during fighting that was caused by something else (we know they used to fight amongst themselves quite a bit; we know that Andruil fashioned weapons presumably from red lyrium - her armour of the void).
I believe Solas (perhaps aided by Mythal in part before she was killed, or maybe by her spirit after she was killed) took drastic action to prevent the fighting from becoming far, far worse, and to prevent more deaths of his brethren. He sealed them away - along with Arlathan although not necessarily in the same physical location as the city - with a view to almost 'freezing' the status quo, either out of desperation or while he intended to search for a solution (perhaps a cure for the taint, or something else).*
So, perhaps the Golden City that people saw in the Fade before the magisters went there was never an actual structure, but a representation of a mixture of different beliefs and understandings that got parsed by the faith of the time - Andrastian religion - but originally had been a reflection of Arlathan, the largest and most impressive of the elven cities (and likely the seat of Elgar'nan). We know the Fade is shaped by the memories and interpretations of those who dream and visit it. When Cory et al do go into the Fade in their physical forms - the first recorded time this had ever happened - they expect to see the Golden City because that's what the collective belief system of the entire continent has shaped there, but what they actually encounter is the sealed away blackened, tainted city of Arlathan. They have no reason to assume anything other than this is the fabled Golden City of Andrastian religion but turned black. They become tainted by it themselves, in such a way that turns them into what we commonly understand now as the first darkspawn. On their return, twisted and confused by what they've seen, the rest of the world scrambles to make sense of it. Andrastian religious belief does the only thing it can - which is exactly what Cory et al did as well: they interpret what they saw as a blackened Golden City, and the rest is history.
*We also know that the people commonly thought of as elven gods could take on a dragon form. It's interesting, then, that the Old Gods are dragons, and that there are seven of them (just as there are seven elven 'gods' sealed away by Solas). However, the elven 'gods' were supposed to have been sealed away in the heavens, while it was the Forgotten Ones who were sealed in the 'abyss' (we know the Forgotten Ones used to dwell in the void/abyss, and the story goes that Fen'harel told them to go dwell there a while, which is when he 'tricked' them). I've seen a convincing theory that relies on the concept of duality that we learn much of in the Temple of Mythal (that each of the elven 'gods' had a darker side to them - Mythal being the god of justice, but also of vengeance, for example). The theory goes that Solas separated somehow the two opposing sides of the elven gods, the good and the bad, the pure and the tainted, and sealed them away independently. That bad, tainted aspect took with it their ability to transform into dragons, and as such is imprisoned as the dragons we understand today, the ones that become archdemons and start blights. I'd go further and suggest that therefore they are already tainted, and Thedas' understanding of how a blight starts is as misinformed as so many other things they believe they know. The darkspawn are drawn to the dragons because they are massive, powerful loci of the taint, far stronger than anything else. Once they reach them and set them free, a blight begins because they follow it above ground and wreak havoc. The dragons remember their war, but little else of who they once were, and so they blindly go rampaging, searching for destruction, with a bloodlust and nowhere to direct it other than at whatever crosses their path. The darkspawn follow. Once the dragon is slain, the darkspawn trickle back down to the abyss/void/underground again, drawn to the next largest, powerful locus of the taint. Once all the dragons are slain (if that ever happens) that doesn't mean there will be no more blights - it just means a decentralised spread of the remaining darkspawn as they have no powerful focus to drive them.
Of course, there is a problem with this theory. Andruil's mosaic said she started hunting the Forgotten Ones for sport. If they were in fact the elven gods (an aspect of them) this could mean a couple of things: she was hunting the other elven gods; or the two aspects of the gods could be separate entities, perhaps as we understand mortal bodies and spirits before they join and after they join, that could come together and separate at will. Either fits with the theory above. However, it also raises another possibility: they were always separate entities, perhaps all were at one point seen as elven gods, but they went down a different path, maybe favouring their dragon forms, and as such ancient elves wrote them out of their history and over time they became interpreted as something else. If this was a war between the elven gods and the Forgotten Ones, Solas' actions could still have been the same. I would suggest they were all still tainted, which is either the reason they warred or a result of their warring, and his sealing them away separately was still a way of 'freezing' what was happening so as to stop it devolving further. The first theory, that the Forgotten Ones were aspects of the elven gods, helps us make sense of why Flemeth/Mythal would want the soul of the Old God -- if her and Solas' plan is to find a way to fix their separation and fix their taint, they'll want to have all parts of the puzzle to Humpty Dumpty them back together again.
*ahem* But yes, the Black City is Arlathan, afaic.