ok so I've read the faqs and it says there are 3 on sable lowlands but I can only find 2, 1 from a pick up off the ground and 1 from the guy near the end of the level (I think it's a raven unit). Apparently there is a black knight unit that charges from the north but I only see 4 excluding him when I play it... (i've tried it 3 times now)
Ok I've figured it out, the black knight comes from the left (shortly after the 4 from above) and the first 2 times I killed him nothing dropped, but the 3rd he dropped one so maybe it's pure luck or maybe it has something to do with the kill order, anyway, hope this helps anyone that was in the same boat I was. :)
While we're at it, you can get several other black knights. You can get the two from Wetinus, either that OR the two from Fair Heights. You can also get the one at the Temple of Berthe, one of them at Tremos Mountains 1, and, of course, Carth. If you can get any at Vert Plateu/the place with Biske, I wouldn't know. If I counted right, that's a whole nother unit of BK's. Great fun in my books.