A pre 1 CY date of incorporation for Blackmoor seems an impossibility. The Great Kingdom was a land power, and land powers grow their borders out and not add to their empires by cherry picking distant lands, like a sea power.
The problem is you're looking at Blackmoor as just another chunk of land on an empire's checklist. In this case, you're right - it has no conventional resources or strategic importance. The know the old "it's
magic!" explanation is a cop out, but for Blackmoor it makes sense. In a fantasy world, magic is a resource. With this in mind, Blackmoor goes from being a poor backwater to one of the most valuable commodities on Oerth, at least to those who know its secrets: mysterious standing stones, the Comeback Inn, the Egg of Coot, the City of the Gods, Wastrian temples, the Land of Black Ice, artifacts of the Ur-Flan and Northern Adepts, etc. Some of these didn't exist pre-CY, but you get the idea.
Also remember the Ur-Flan were still a threat at the dawn of the GK. In -108CY they tried to assassinate the King of the Aerdy. Maybe the Aerdy conquered Blackmoor in -20CY to stamp out the last vestiges of Ur-Flan resistance. Maybe Ranial the Gaunt had WMDs. Maybe the Aerdy battle mages saw Blackmoor's magic as too dangerous to fall into the wrong hands. Maybe Blackmoor's conquest was the pet project of some rich price who went on to develop one of the artifacts described in
Ivid. Rauxes was once the spelljamming capital of Oerth, maybe that helped. Maybe they
did teleport.
Trying to force the Great Kingdom's expansion into a linear series of conquests based on each land's distance from the capital is incredibly boring. It's not even realistic. Invent reasons for each land's conquest besides acreage, proximity, and fertility and GH has a lot more depth. Suddenly Urnst becomes a powerful rival second only to Keoland. As for their lengthy independence, maybe Urnst was a valuable military ally, doing the Aerdy's bidding to avoid absorption. Maybe they were crafty negotiators. Maybe the Bright Desert dervishes and Abbor-Alz barbarians were more powerful then and the GK wanted a buffer. Maybe the GK feared the Maure or the elves of the Celedon. Maybe the GK preferred to use Urnst as a Rhennee dumping ground.
The GK apparently controlled both Ferrond and Blackmoor militarily;
How did they get there? The lands north of the Nyr Dyv are pretty wild even in the 570s. They must have been absolute wilderness prior to -20 CY.
You're talking about the Ellis Island of the Oeridian migrations. These lands might have a had a Wild West vibe, but they were settled long before the eastern Flanaess. We only know that the Northern Reaches were lawless under the under the rule of Viceroyalty of Ferrond, post 1CY. Before that, they could have been more civilized. Also, we barely know anything about the history north or east of the Shield Lands (i.e. the Bandit Kingdoms) since they were roped into Iuz's chapter in the LGG.
