I have no info on Black Knifes at all - animal-world.com listed them as -
"The African Knife Fish Xenomystus nigri was described by Günther in 1868. They are found in many of the coastal rivers of Africa including the Congo, Nile, Niger, Ogooue, and Chad River basins. Most of the specimens that make it into the aquarium hobby are exported from Nigeria.
The species is listed on the IUCN Red List as Least Concern (LC). But it has a wide distribution with no known major threats. Other common names they are known by are African Brown Knife Fish, Black Knife Fish, Brown Knife Fish, Brown African Knifefish, African Black Knifefish, and Black African Knife Fish."
So perhaps they are the same species. I have 3 on order to find out.