Was there an ancient, technologically advanced civilization in Blackmoor? Tavis Alison wrote a piece a long while back on the Apocalypse Trope in D&D, and in that article he mentions the gigantic pipe organ found in the original (1975) Temple of the Frog. The pipe organ was Steve Rochford's (Saint Steven) idea, but one that Arneson happily ran with. To explain the existence of a mechanical modern-era musical instrument in an otherwise medieval setting, Arneson called it the last surviving example of its kind, detailed only in a "cryptic manual on artifacts found amongst the volumes in the Library." The pipe organ then, is not some alien technology installed by the visitors from outer space who had taken over the temple, but a remnant of a distant time when there were builders with the technical skill to produce such a thing. Does that mean Blackmoor once hosted a more technically advanced civilization? Tavis wondered the same thing, so a year before Arneson's passing, Tavis asked him that question at a Gen con. He asked "...whether this pipe organ implies a previous high-tech fantasy civilization or whether Blackmoor is a post-apocalyptic Earth." Typically cryptically, Arneson answered "yes" to both, implying that he wasn't fully committed to either view, and that may be, but we can say with surety that back in the 1970's when they played John Sniders Star Probe games or Arneson and Barker's crossover Tekumel games, Blackmoor was considered to be a place distinctive from Earth.
Now a single reference to a pipe organ may seem like pretty thin sauce, you might say, ah but there's more juicy bits to be had.
For example, we have this description in the FFC of the throne room of the Egg of Coot, "...which is dominated by a huge old world artifact said to be an ancient war machine." (77:18)
What is this "old world" of which Arneson speaks? It's a mystery of course except that undeniably there was once a civilization capable of building advanced machine artifacts, somewhere and somehow tied to present day Blackmoor.
The secrets of such technology might not be entirely lost either, at least, not all. William of the Heath, an original Blackmoor character played by Bill Heaton, had among his possessions three blue items: the magical sword Blue, a mechanical horse named Bill, and mechanical blue armor. Once again, neither the horse nor the armor appear to be space alien technology. They are items with a history, having once been in the possession of a wizard and a dragon, and neither rely on rechargeable batteries or power packs as Arneson's alien tech usually does. The text does not make it clear, but seems to hint the wizard made all three of these items, and if so he must have some bits of ancient knowledge. There's really nothing definitive on the exact nature of the armor, but we do know the horse "never seems to eat anything and drinks lamp oil." Drinking refined kerosene pretty clearly implies a mechanical creature, not an electronic one.
It's also worth noting that the d20/3.5 era Blackmoor books freely dip into steampunk, as exemplified by Clock and Steam wherein we find that "the titanium charger represented the peak of technological development in that it nearly perfectly replicated a horse, all the way down to its behavior and mannerisms, but improved upon the technology that powered mechanimals by making it a valuable companion in combat." (p124) To my ears, the mechanical titanium charger seems meant to be an homage to "Bill".
While it is certain Arneson did see Blackmoor as having some sort of a lost "old world" advanced civilization, he never really pursued the idea beyond references like those above or to ambiguous "Technical Manuals" and "ancient books and manuscripts". Even the steampunk creations in the 3.5 era Blackmoor books were attributed to gnomish and dwarvish inventors, perhaps a bit inspired by alien tech, without mention of an ancient, advanced civilization.
However, the idea of a lost ancient technological society in the region of Blackmoor was picked up by Greyhawk writers - eventually. Perhaps the first hint of this was in The Living Greyhawk Gazeteer, "It is not known what, if any, civilization existed in the far northern land before the Ice claimed it, but the rumor of evil in the north was old even when the Oeridians and Suel were new to the Flanaess." (2000:34)
However it was Wolfgang Bauer who really ran with the idea in two adventures published in Dungeon Magazine The Land of Black Ice (#115) and The Clockwork Fortress (#126).
In the former adventure, we find a nimbleworks who is "a strange construct from an ancient realm that predated the Black Ice." (p32) In the latter an entire fortress of this ancient civilization is detailed, and we learn that long before the Oridian migrations, "...a small fiefdom in the northlands reached the heights of civilization - its craftsmanship and knowledge of artifice were unmatched." (43).
Bauer goes on to conflate this ancient civilization with the City of the Gods in the second adventure, but we know that label usually refers to a crashed spaceship, not a lost civilization and attributing it to an incredibly old civilization causes a dating conflict with the information from the Codex of Infinite Planes in Eldritch Wizardry. Of course, there certainly can be more than one City of the Gods, or the name of one could have been carried over to the other by people who didn't know the difference easily enough.
Whatever the role of the City of the Gods may be, the advanced ancient technology of lost civilizations found Bauer's two Blackmoor adventurers isn't coming out of nowhere but is rooted in a tradition that goes back to the earliest days of our hobby.