"Strictly speaking, Zeitgeist cannot mention Mystara in their Blackmoor products, unless such a deal was struck when the rights reverted back to Mr. Arneson. Though the two were connected in the Mystara setting as published, Zeitgeist (that is, Mr. Arneson) should only have gotten back the initial rights to Blackmoor. Again, I am unsure as to the status of that, so I could be wrong. WotC was very generaous with the deals they made with Mr. Gygax and Mr. Arneson, so potentially, Mystara could be mentioned in passing, though that would profit Zeitgeist not at all, save with us Mystara grognards. The situation is completely different with the Blackmoor of Greyhawk, as Greyhawk is still owned by WotC, and the Blackmoor as presented there is not the Blackmoor of Mr. Arneson, regardless of connections that fans might make.

Now, should Kenzer actually do a Mystara setting, likely "Blackmoor" will not be mentioned, as the setting is not a WotC setting any more, legally speaking.

[...]

Provided that the Product Identity is properly guarded in the Blackmoor D20 book, no one can use "Blackmoor" as Zeitgeist designed it in any product, whether they have a license from WotC or not. Kenzer's license to use the old D&D/AD&D material, as far as I know, and I am fairly certain on this, extends only to the extent that they use it in a HackMaster product. They cannot use it in a D20 product, or an AD&D/D&D product, as those are separate licenses. It is like the difference between having a license to reproduce a movie on VHS, but not on DVD...

So, provided the language in the Zeitgeist Blackmoor D20 book is proper concerning the nature of Product Identity versus Open Content, there is no problem with this..."