If you want to form Belah'dian headcanon, I'd warn against getting specific unless you're perfectly fine uprooting it when more information comes out.
If not, be vague.

Every time a discussion comes even remotely into Belah'dian territory, be it in game, in these forums, or in an interview, the subject is gently nudged away from it. What you can summarize is that it's a Sixth Astral Era predecessor of Ul'dah and Sil'dih, it was likely formed by the surviving black mages of the Fifth Astral Era that we don't know much about, they built on top of existing ruins at Qarn, and Square Enix intends to fill in some of these blanks, someday. More likely than not, the new information will focus on the black mage city of the War of the Magi, but it may end up contradicting something you've invented in your fanon.