Pencil Fish coloring has changed

I've had 7 Pencil Fish in a community tank for 18 months. About 2 months ago the color on one of them changed so that the last 1/4 of it's body is black. It wasn't a gradual change. It's behavior hasn't changed. It still eats and swims with the the others. The remainder of it's body might be lighter than usual. I haven't noticed anything to cause me to be concerned for this fish, but I'm curious if others have seen this or if anyone has an explanation.

The pictures are not very good, but you can see what I'm talking about.

 

I have never seen that in any of mine but I found this thread in another forum, the description sounds similar, unfortunately there wasn’t any satisfactory answer.
Black patches on Three-lined pencilfish (Nannostomus trifasciatus)

 

Thanks for the link. Yeah, sounds like the same thing. I think the fish is fine. I'll update this thread if there are any new developments.

 

Actually...

Pencilfish have some interesting social behaviour. With my N. eques I can see 3-4 different colourations depending on mood, social status AND surrounding light.
Dominant males are relatively dark, almost black with a golden reflecting lateral line, low ranking fish tend to have a white belly and brown upper body half, stressed fish have the night-colouration of being almost transparent white with 2-3 stripes across the body axis.

And exactly this night/stress colouration can easily shine through in mildly stressed fish.

Here the difference in N. eques for example.

 

Yes MacZ, I've noticed that too. The first time I noticed a male colored up all dark I took a double-take.