They must be tank bred. I only know of some bloke breeding BGK's in brisbane, but dont know where or who. www.aqualandpetsplus.com Then scroll down the page on the right to find Black Ghost Knifefish, and it will have a small report of that.
I just came across this link tonight wish I could find a better pic. But the 2nd pic down on the left is labeled polkadot blackghost and if you go to the bottom there selling them. They look very cool anyone ever see these. Here is the link.