I understand that by the time Zeitgeist Games was publishing Blackmoor, the licenses for Blackmoor and Mystara had been legalistically separated. However, for those of us for whom Mystara and Blackmoor are inseparable at our game tables, I have made the following Mystaran creature substitutions to "The Dungeons of Castle Blackmoor" product when it calls for creatures that are definitely NOT Mystaran.

Instead of duergar, I use Modrigswerg dwarves.

and instead of aboleth, I use kopru.

I think these substitutions keep the spirit of the written dungeon adventure while also staying true to the classic Mystara/Blackmoor continuity.