Can you post a picture? There are diseases that can cause parts of a fish to turn jet black, but you'd notice if your fish was sick enough for that to be the cause. A picture and your tank specs and water test results would really help.
No, the whole freakin' thing was black, and I thought, "maybe someone put a new fish in there (for some ungodly reason)", but, no, it was the fish, I can tell by the "feelers".
Everything is normal, no detectable nitrates or nitrites, not soft water, and a rather high pH (8.0).
I've never heard of that, except with discus - when they're feeling miserable, they turn a sickly charcoal colour, I've seen it with one in a (!) community tank at my LFS - or in cases of disease.
I just checked Untegasser's 'Handbook of Fish Diseases' and if the fish has a swelling at the roof of the mouth or operculum, it can be a sign that a thyroid tumour is pinching the nerves responsible for colouration (no kidding, it says that!) and adding iodine to the tank will cure it. If there's no swelling, and the only symptom is that the fish has turned completely black, it can be pinched nerves caused by another type of tumour, and isn't curable. So in other words, your fish COULD have cancer or a goiter.
One of my two Gouramis turned dark, then died. A couple of days later, the OTHER gourami turned that same color, so I removed it and examined it... I should have taken a picture, but I will describe it as best I can:
It wasn't BLACK, but it was very dark, like charcoal, and it "faded" down to the natural blue color, but it looked like "drippings" of color down the sides, the charcoal did.
The second gourami is back to a bright baby blue, so I am not worried any more... too acute a condition for a tumor, I think.
*scratching head*
That is the strangest thing I've seen in a long time... I'm going to do some gourami research for my own curiosity, if I come up with something I'll let you know.