Apparently there is a shirt for sale with the logo "It started with Blackmoor - 1975". The date indicates they are referring to Blackmoor supplement II for OD&D, which was published in late 1975, but that makes the caption a bit strange and I don't think there is a similar shirt for Greyhawk Supplement 1, also published in 1975.
However odd the shirt may be, it brings up a good occasion to point out things that fans of Blackmoor often take for granted to the point of not realizing that not everyone knows them. So, at the risk of pointing out the obvious to some, here is a list, no doubt incomplete, of things that actually started with Blackmoor:
The Dungeon
The 1960 edition of the classic Funk & Wagnalls standard dictionary of the English language defined dungeon as "A dark underground prison." Prior to Dave Arneson sitting down with paper and pencil and sketching out 6 levels of chambers and passages descending below castle Blackmoor, a dungeon was thought of as little more than a dank hole in the ground.
The Castle
Of course, castles in a fantasy game are a given and it was only natural for Arneson to use his lovely model of castle Branzoll in his Northern Marches game, but by doing this, and placing his extensive dungeon under it, he created the Castle/dungeon trope ubiquitous in D&D. In the real world, castles do not have multi-level underground labyrinths. In D&D they all do thanks to Dave.
What Arneson envisioned was new, but not unfamiliar, drawing on the images of caverns, catacombs and secret passages in monster movies, and the labrynths of mythology to create an amazingly complex vertical and horizontal underworld peopled with monsters.
The Home Base
Also above the dungeon, the town of Blackmoor - apparently based on the wooden model of Cuidad Rodrigo as discussed in other posts - was almost certainly not initially conceived of as a home base where players could recharge and resupply between adventures. I suspect rather we see Arneson once again replicating his beloved classic horror films. For what is a dark castle without a nearby village - a place where the protagonists can be forewarned of the danger and from which can spring a mob of angry villagers to storm the castle? Of course there is also the fact that when Arneson began the Northern Marches campaign it was conceived of as a development of their "Braunstien" games which were always set in a town or city. Nevertheless, functionally the town served exactly as a "home base" in D&D terms and the players, and for that matter Arneson, naturally understood the advantages the town provided for them.
The Tavern
"You meet at an Inn". Surely this is the most used trope in fantasy role playing games ever and of course it is a trope that starts with Blackmoor. Inspired by a real Come Back Inn in the Melrose Park neighborhood of Chicago, Arneson used his ComeBack Inn in the town of Blackmoor as a place for his players to gather to plan activities and gain information, just as thousands of GM's have after him.
The General Store
There is no particular reason why characters in an adventure RPG should have to go shopping, yet it is a fundamental expectation of D&D. Early in the Northern Marches campaign, Arneson created the general store, price lists for weapons and equipment, and placed Dan Nicholson in charge as "The Merchant. This sort of "local functionary" character was typical of their previous Braunstein games and a role Nicholson would have been familiar with. He quickly took advantage of his de-facto monopoly to set up an underground shakedown network to ensure all the profits stayed with him - but that is another story. In D&D, merchants and store-clerks were relegated to the GM as NPC's yet the activity of going to the store to haggle with the merchant over goods and services remains a typical part of the game.
The Cleric
Blackmoor had no classes, excepting, of course, all the classes Blackmoor had. Say what now? There were no classes as we know them in Blackmoor at all. All characters were statistically the same regardless of what they did and a person could be pretty much whatever they wanted. Thus at various times Dave Megarry played a thief, Dan Nicholson a merchant, John Snider was an imperial Inspector, etc., etc. These roles didn't have mechanical components associated with them the way D&D classes do now, but differences began to arise that made some roles stand apart. In particular wizards and priests grew to be "different". For wizards the main difference was in how they advanced in spell levels and possibly in how they gained experience points. For priests, the differences were more distinctive. Priests gained the ability to cast healing spells and even to resurrect the dead. This last ability appears to be tied to the deliberate and ultimately fatal risking of Greg Svenson's character by the other players while Greg himself was away in the summer of 1972. As this death of a beloved character was not really fair to Greg, Arneson invented the idea that "The Great Svenny" could be resurrected by the clergy. Subsequently a lot of resurrections occurred in the Northern Marches. Later, as vampires plagued the land, Priests also gained some abilities against undead - presumably skill at turning them. These distinctions set priests apart as effectively a different class.
Non-human Player Characters
Some of the earliest Blackmoor adventures were hexcrawls wherein the characters played themselves as transportees to the magical Northern Marches. However players began to also play local characters and some of those local were not human. John Soukup, for example played a Balrog, Phil Grant played an elf prince, Walter Oberstar played a dwarf, Mel Johnson played a hobbit and Fred Funk played an orc. While today it is simply taken for granted that fantasy RPG have all sorts of non-human player character options, in the early 1970's this was an entirely novel idea.
Gothic Horror and Dune Salad
D&D players today take it for granted that they might face a medusae and a vampire in the same adventure - that's all due to Arneson's kitchen sink approach to the game. If you look at CHAINMAIL, the fantasy wargame booklet that provided many monsters for the Northern Marches campaign, you will find only the classic monsters of Greek and Norse mythology with Tolkien flavoring added. Arneson once again drew on his love of monster movies to mix such disparate monster types as Greek medusae, Gothic vampires, Fremen-like desert raiders and alien blobs in the same game. Not only did such a crazy mix work, it worked brilliantly and is the basis of every monster manual and every D&D campaign since.
Experience points for Killing Monsters and Treasure
Lastly, let me mention something that didn't quite begin with Blackmoor but for which Arneson is often credited - gaining levels of experience. Arneson may well have coined the term "levels" and "leveling up" for growing more competent and powerful by stages, but in truth, the concept of advancement was a core part of the Strategos N rules for units. "Green" units of recruits who survive a certain number of battles will improve to veteran status and can even become Elite. Similarly, in Duane Jenkins Brownstone game started a few months just prior to Arneson's Northern Marches campaign, a character could go from being a nobody to a somebody over the course of several games. Jenkins doesn't seem to have had a unified mechanic for advancing player, and some roles appeared static, but Arneson seems to have solved or at least simplified that problem by awarding experience points for killing monsters and finding magical treasure. He also created a system for wizards to gain new spells. It thus became a goal for all characters to "level up" to become better at killing monsters and to gain a bigger role in the game.
Supplement II Blackmoor by TSR didn't really start much, but the caption of the shirt isn't wrong. It did all start with Blackmoor.
