Some of those Platyrhincus down there are really dark fish. They are actually well known for it. I would not consider it a rare trait but certianly a nice one... (Hell getting one not from a ditch farm / catfish farm or asia is benificial in my mind.) If these are coming from the wild then they are still better than 80% of the typical LFS fish that are often sold.
I would be Leary of thinking these particular fish are much like the Melanistic Longnosed that pop up in certain populations though. (Of coarse most tend to all get dark in spawning... there are some however that regularly produce Black gar at all seasons)
The variability of these fish is amazing and they are also chameleonlike when it comes to dark and light.
Interested in the photos of the fish when you get them...