I do a lot of electrofishing for work and have seen wild and hatchery "black" trout. The fish can lighten and darken themselves based on light and stream conditions. In the winter we see a lot more dark trout which is most likely caused by the lower light levels. When the hatchery I used to work at emptied its drainpipes for cleaning a big old monster bow was found. The thing looked almost albino and kind of resembled a koi with how fat it was. After a while in the light it began to darken up to normal colors. I left that job and never got to see what it ended up looking like.

Adam