Blackmoor, Greyhawk, and Mystara
Originally, when first published in the "First Fantasy Campaign" (published in 1980, the same year the World of Greyhawk Gazetteer was released) Blackmoor was set in the City State world of Judges Guild. The City State maps were 5 miles her hex; thus, the Blackmoor map, at 10 miles per hex, was the size of any four Judges Guild maps. The lower left-hand quarter of the Blackmoor map was designed to just north of the Campaign Map 3: "Valley of the Ancients" map from the City State setting. (Note that later TSR left in the reference to the "Valley of the Ancients" in the DA1 Blackmoor setting map! The nature of the valley was, of course, altered completely in DA3: The City of the Gods.)
Nothing ever really came of the "First Fantasy Campaign" book. There were no followup adventures or source materials. It was felt by many to be inferior to the Greyhawk Gazetteer, and, for purposes of actually running a campaign, it was. The FFC booklet was basically little more than a mish-mash of campaign notes (and a few useful rules) from Dave Arneson, though the map was cool. Today, it is a wonderful bit of nostalgia, plus it gives a lot of insight into the campaigns of the day...
Then, in 1986, when TSR got the license from Arneson to publish Blackmoor, they ret-conned Blackmoor to tie in with the Known World setting (which was not then known as Mystara).
Blackmoor appeared in four module/sourcebooks:
DA1: Adventures in Blackmoor (1986)
DA2: Temple of the Frog (1986)
DA3: The City of the Gods (1987)
DA4: The Duchy of Ten (1987)
There was supposed to be a fifth book, DA5: The City of Blackmoor, that never got released (nor do I know if it was ever actually written.)
The DA series Blackmoor setting actually used the map from the original "First Fantasy Campaign," published by Judges Guild. The scale of the map was changed from 10 miles per hex to 24 miles per hex; otherwise, the only differences were in coloration and style of presentation. The map was a perfect match for the original from JG. It even used the Judges Guild hex numbering system, though for some reason the count was off (JG startes with Hex 0101, while the TSR map started with 0501).
Blackmoor was cast as the pre-history of the Known World of Thyatis, Karameikos, and so forth. The DA series of adventures was said to have been set 4000 years prior to the "modern era." Originally, in DA1: Adventures in Blackmoor, the Blackmoor era was said to have been destroyed circa 3000 years prior to the "modern" era of the Known World... but then, at the time, the actual time frame of the Known World had not been completely established. Several modules from that period attest to the Thyatian Empire being only two centuries old, at best, and no two were really coordinated on their dates or even as to the nature of the Known World.
The first Known World Gazetteer, The Grand Duchy of Karameikos, placed the destruction of Blackmoor at 2000 BC (Before Crowning of the Emperor of Thyatis), with the "modern" era being set at 1000 AC (After Crowning of the Emperor of Thyatis). This fit in perfectly with the DA series materials.
The "actual" date of that cataclysm, 3000 BC, as later utilized in Mystara Canon, was first set in the second Gazetteer, The Emirates of Ylaruam. This was also the first place where it was referred to as the "Great Rain of Fire," hearkening back to the "Rain of Colorless Fire" of Greyhawk. All other subsequent Gazetteers used the 3000 BC date, except for the "Karameikos: Kingdom of Adventure" boxed set, which, due to editorial mistakes hearkening back to the original work from which it was derived, used the 2000 BC date.
Nowhere in the DA series was the setting ever tied in with the Greyhawk setting, though they legally could have done that. Even at the time, however, there was, due to legal contractual issues, a conscious effort on the part of TSR to separate D&D from AD&D, thus, no such thing would have occured.
The DA series ended in 1987 with DA4, however, the Known World Gazetteer line continued to be published from 1987 through 1991. The Challenger series then ran through 1993 or thereabouts, and, finally, the 2E AD&D editions of Mystara were published through 1995, IIRC. The 2E editions continued to mention Blackmoor, though apparently, only though editorial mistakes. I would presume that the Blackmoor license had either run out, or they were trying to divest themselves of the entanglement.
Today, Zeitgeist Games is resurrecting Blackmoor for d20, Necromancer Games is reviving the Judges Guild City state setting for d20, and Kenzer & Company will be releasing the HackMaster version of Mystara. None of the three, as far as I am aware, will be related to the others in any way, nor to Greyhawk.
James