Both Blackmoor (Mystara) and Greyhawk (Oerth) evolve from the same original map that both Arneson and Gygax used in the 1970s for Proto and Original D&D.
Each world, Mystara and Oerth, evolve differently as they each fill out the unknown space beyond.
The protomap is extensive, including Blackmoor, the great lake Nyr Dyv, and other places in the periphery. The map represents something like a continent.
Iterations from the earlier protomap often preserve recognizable features, but there can be spacial distortions for far away features − to a degree that reminds one of reallife medieval maps! A small region on the edge in the southeast can become a vast empire far away. A regional forest might become local.
For fun, I compare the locale of Blackmoor in the maps of Mystara and Oerth.
My methodology is to locate corresponding locales. Note, the Oerth map is more vague and the Mystara map more specific. Even so, the "Archdutchy of Blackmoor" in Oerth correlates with places near the City of Blackmoor in Mystara.
Oerth location: associates Mystara location
• Egg of Coot: Egg of Coot
• Mosshold: Maus
• Ramshorn (Castle): Ramshead (City)
Where the Oerth map vaguely describes as wetlands, the Mystara map details a patchwork of drylands, rivers, lakes, and wetlands. For example, where Oerth has the Egg of Coot at the edge of a wetland, Mystara has it on the coast of a brackish estuary. Presumably the water is shallow and the coastline shifting in places.
When we look at the triangle of these three locations, Egg of Coot, Mosshold, and Ramshorn, both the names and the distances correspond between Oerth and Mystara. Near this triangle, locations and distances correlate serviceably.
The location of the original "Old City" of Blackmoor is at the center of this triangle.
Interestingly, the "ruins" of Blackmoor according to Oerth appear to actually be the ruins of the City of Ringlo according to Mystara. It situates between the Redwoods and the Elven Forest. With regard to the Oerth map, it should probably be understood as "the ruins of the Archdutchy of Blackmoor", and specifically Ringlo, rather than the ruins of the City of Blackmoor itself that is elsewhere.
I edit the Oerth map below to show the location of the "Old City" of Blackmoor. Blackmoor, Egg of Coot, Mosshold, and Ramshorn appear as red dots. I add some terrain details from Mystara that cover an entire hex space in the Oerth map. There are areas of wetlands that are mostly underwater and boatable. Presumably where Oerth has wetlands and Mystara has mountains, these are high-altitude wetlands. The desert within the Cold Marshes is the legendary Valley of the Ancients. I represent the Firefrost Channel flowing from under the Black Ice. Much of what is the Burneal Forest in Oerth is the Plains of Hak in Mystara, possibly a flatland patchworking grasslands and woodlands. Plus, there are a few Mystara cities to get a sense of whats-where.
The Archbarony of Blackmoor
(edit. Yaarel 2021)