Blackmoor was Dave Arneson's original campaign, and its origin is well-covered in Jon Peterson's Playing at the World. It was the cauldron of creativity where he fully distilled a game featuring individual characters as opposed to battling armies. Many of the features that would become Dungeons & Dragons began in the Blackmoor campaign.
Of that, as a young man, I knew nothing. In the tender year of 1975, Freshman year of college, I knew (through TSR and the fledgling Dragon) that there were two big campaigns - Gary's Greyhawk and Dave's Blackmoor (Tekumel and the Empire of the Petal Throne was larger than either, but it had its own game rules, and as such was an alien creature). I assumed that since I had acres of maps for my own new campaign, there had to be rafts of material of the two original campaigns just waiting for print (oddly enough, years later, that was EXACTLY the case with Ed Greenwood and the Forgotten Realms, but that's a different letter). Greyhawk had gotten its own small book, which said little of the World of Greyhawk itself, and I was already running D&D by the time the Blackmoor booklet turned up.
|An original Blackmoor map|
And it was a bit disappointing for someone looking for more details on the world itself. While the Greyhawk booklet launched a lot of new concepts into the game, Blackmoor was oddly less cool, as if the concept of what made up D&D had already gelled. It was the sophomore slump of a booklet. It brought us the official Monk, but also the problematic assassin, a hit location system that was never used, a bunch of giant creatures or specialized critters (like the Sahaguin), and much of its running time was dominated by the Temple of the Frog. Here was the first example I had of a specific dungeon, and I found it a TPK (in particular that frog spawning pool with, like 1,000 giant frogs, which were overpowered to start with). It succeeded in making me more comfortable with my own extremely lethal early dungeons, but didn't get much that felt new. Only later in life did I get the idea that this was small sliver of the larger campaign.
Blackmoor itself, after all these years, still feels portable to me. It was Arneson's original campaign, and as such was just as large as it needed to be. It was welded to the north of the C&C group's Great Kingdoms, the prototype of what would become Oerth, then made independent in the First Fantasy Campaign, then got a name check in the World of Greyhawk, then was re-imported into the D&D Known World, and continues on in the hands of others. For such a foundational world, it feels like it has deserved better. Here's a site with a lot more backstory than I can give here.