Just saw this. Two possibilities: it is natural (I will explain below), or there is something wrong with one fish. I can't detail the latter option without seeing a photo, so I'll assume the more likely first option and explain.
Hyphessobrycon megalopterus, the Black Phantom, is a characin with sexual dimorphism in the mature fish; that means, male and females appear different externally. The male is basic grey that during displays and spawning darkens to near black over the entire body. There is no other colour, other than a slightly metallic blue/grey luminescence surrounding the humeral (shoulder) black patch behind the gill covers.
The female is reddish or pinkish-hued over the body, with bright red pectoral, ventral and adipose fins. The male's fins are all black. In mature fish kept in a group of males and females, this visual distinction is very apparent. For an illustration, check this fish's profile in our Fish Profile section; I just updated it last week with a photo of a female (on the left) and a male (right photo) that should make this clear. Here's a direct link to the profile: http://www.tropicalfishkeeping.com/p...phantom-tetra/
If this hasn't answered the question, post a photo and I'll take a look.