A black Fimir...
I bought a European version of Keller's Keep as it was the cheapest I can find on Ebay at the time, though I have an American base set. I've noticed, at least from that box of Keller's Keep, that the greenskin figurines were made of a slightly lighter and dryer shade of green. The difference is hardly noticeable through a picture, but obvious with the right lighting.
From what I can tell by these pictures at least one of the black fimirs has a broken bar on the base, meaning the orc and goblins connected by these bars would've been black as well.
Although more black colored figurines appear out of Europe (Against the Ogre Hoard & Wizards of Morcar), but it does indeed look more like an American base. Although it's possible that the original figurines for the playtesters were black and this was later changed to dark green; the American version was never playtested, but simply adopted and adapted to a higher range of difficulty (Had they playtested the original in America they would've found the Trial to be too difficult with monsters having more then 1 body point).
So I'm going to suggest that during mass production of the American version they ran out of the green mold and ran a few using black instead. Maybe they tried to recall it later, but they missed a box or two. Either way this would be a very rare find.