OK, so a "Black Diamond" is a Potamotrygon leopoldi? So it is a Leo. It is just a glorified Leo. Beautiful none the less.
My thoughts are, it is just a nickname for a "strain" of Leo. Similar to the way Discus have several different "strain" names to differentiate the color types. Maybe this will change with DNA classification as I believe they have not done DNA classification for many ray species, if any?
I do think vendors selling the "black diamond" rays should make this more clear (maybe they do?) but I am sure that if they do not, it is because people might not find them as valuable if they were listed as a Leo. I do however notice that H2O has them listed as a Leo. I have heard great things about that vendor so that is no surprise.
Either way they are just a Leo, not to say they should be considered less valuable. It is a case of beauty is in the eyes of the beholder/buyer. For instance, I personally think a great Henlei is more beautiful than just about any BD I have seen but that is just my opinion.