When Arneson drew his original map of the Northern Marches and sent it off in a letter to Rob Kuntz in March of 1971, it is unclear what he intended.
Yes, he drew it as a setting for fantasy war games and his "medieval Braunsteins", but it isn't clear if this map was purely an independent creation, or was drawn with the intent to fit within the Castle & Crusades societies' Great Kingdom campaign.
Mention was made in the C&C broadside Domesday Book #6 (August 1970) that a campaign world for medieval battles was being created along with a map. However, this now famous Great Kingdom map didn't appear until Domesday #9 published sometime prior to June 1971, when Rob Kuntz' position as King was temporarily usurped by Gary Gygax. This issue also announces the forthcoming CHAINMAIL booklet, however, the GK map doesn't actually claim to be intended for the Perren and Gygax LGTSA/CHAINMAIL rules. Instead, the map is provided for "A(vallon) H(ill)-type play as well as aspects of Diplomacy."
Accompanying the map is 3/4 of a page of text in which the organizational structure of the Great Kingdom and neighboring kingdoms is outlined. Specifically, details on the various noble ranks of players and how many castles each rank of nobility controls along with brief comments on troop strength - 1 garrison per castle plus levees. The Paynim Kingdom is cited as being of 150% greater strength than any other, presumably representing the great enemy of all.
Arneson's letter makes no mention of the GK map. The players cited and their troop strengths includes no mention of ranks or castles, or garrisons, nor is the Paynim Kingdom mentioned. In fact, the letter doesn't even reference the Great Kingdom. Instead "The great EMPIRE OF GENEVA" which, while today we might easily read as a euphemism for the GK, is likely nothing more than a flowery fantasy reference to the Castle & Crusades organization itself.
It seems very likely therefore, that Arneson's "Northern Marches" map and the campaign he describes in the letter were done independently initially, and not as an attempt to flesh out a corner of the C&C map or with the intent of fore-planning for the C&C campaign. When Arneson enclosed this map, along with a letter to Kuntz in March of 1971, he describes something he has already set up and possibly even run games in. The tone of the letter is most consistent with an existing campaign with no particular connection to to the planned C&C campaign,
Given that this letter was written prior to the release of CHAINMAIL and it's Fantasy section, the Northern Marches as an independently conceived undertaking is also perhaps the only way to sensibly understand Arneson's comment that his "medevil project is... partially fiction" and has something called "the accursed lands of the unholy RED WIZARDS COVEN".
Arneson appears completely unaware of both the soon-to-be-released fantasy rules in CHAIMAIL and Great Kingdom map. His description of the lands surrounding the Northern Marches, including the Skandanarians to the north, the Picts to the SW and Eraks to the east is incompatible with the layout of the GK map. If Arneson were in fact aware of the GK map in March of 1971, it's very clear he wasn't trying to locate his Norrthern Marches on that map.
There has been, I think, a general assumption that as Arneson created Blackmoor, his intent was for his gaming in this locale to be tied to the Great Kingdom. In fact we appear to be told just that in the introduction to D&D in Men & Magic, where Gygax states:
"Dave Arneson decided to begin a medieval fantasy campaign game for his active Twin Cities club. From the map of the "land" of the "Great Kingdom" and environs -- the territory of the C & C Society -- Dave located a nice bog wherein to nest the wierd enclave of "Blackmoor", a spot between the '''Great Kingdom" and the fearsome "Egg of Coot".
Technically speaking, the above doesn't preclude the idea that Blackmoor was first created elsewhere and moved into the GK map, but a reader would hardly be expected to intuit that.
Nevertheless that appears to be exactly what happened, for there is no question but that all parties concerned came to think of Blackmoor and Greyhawk as connected locations on the same continent - to the point that cross over adventures took place, including The Great Svenny flying to Greyhawk and Robilar and Mordenkainen traveling to the City of the Gods. There's many such references to point to.
So what happened? We might guess some reasons. Perhaps, as seems likely, Arneson received DB #9 after writing the letter to Kuntz, and decided to recast his "medevil project" onto that map instead.
In any case, what Arneson did next was particularly significant, in a way that has largely gone unrecognized. Arneson drew a new map.
He drew a new map within the area of the GK he had claimed. With this new map, Arneson choose not to start from scratch but instead recast the world he had already created. This suggest fairly strongly, that Blackmoor was more than just a few scattered ideas, it was something enough time and energy had been invested into to want to preserve.
This was the new map he created, as published in the FFC:
And it was intended to fit here:
We don't know exactly when he did this, but it seems to have happened very early on. Furthermore - and here is perhaps the surprising bit - the entire Blackmoor campaign of 1971-1976 was set in and around this Blackmoor/Greyhawk world map, not the now-more-familiar maps like this:
Observe:
"His nearest neighbor is Sir Jenkins who rules the Northern most march of the Great Kingdom which rests on the actual frontier with the Egg of Coot." p25
Both the original "Northern Marches" map and the current Blackmoor map show plenty of water separating the Egg from Glendower where Jenkins ruled along with the rest of Blackmoor, but one only need observe that the Egg of Coot never had a Navy during all those invasions to confirm that those maps don't fit the text. Here's another:
"WOLF's HEAD PASS: This area lies some five miles to the North East of the Castle along the only road that leads to the Southern confines of the Egg of Coot. Beyond the pass there lies an extensive no man lands of some twenty miles before the southern reaches of that evil area are reached." p26
The road thus described can be easily seen on the sketch map:
When Arneson decided to publish the First Fantasy Campaign, he had little incentive to maintain the Greyhawk/Blackmoor connection and when Bob Bledsaw drew those excellent maps that came with the FFC, he went back to Arneson's original Northern Marches map. The obvious differences in the bays and waterways were hand-waived as "Sinking Lands". Blackmoor had gone back to it's roots, but in doing so the geographical context of those early games was significantly obscured.
