Both Blackmoor (Arneson) and Greyhawk (Gygax) expand from the proto C&C Society kingdoms map. There are inspirations from North America, as well as Europe including UK, Scandinavia, and Netherlands, and wider Asia.
Here is a Gygax-Kuntz 1970s version of the "Great Kingdom" map.
Blackmoor, the Egg of Coot, and the Ten Duchies detail the southwest corner of the Great Bay/Hudson Bay/Blackmoor Bay. Greyhawk details the southwest corner of Nyr Dyv / Lake Superior.
This is an Arneson-Megarry map, drawn from memory by one of the players in the Arneson campaign during the 1970s.
While there are distortions in distance, many of features are recognizably shared by both Blackmoor and Greyhawk.