The black koi is a beautiful fish when you have the right background to showcase this fish against, and even when you have a dark background, these fish are neat additions beacause their presence is usually seen as a shadow gliding over and on top of your other fish.
Black koi are the descendants of the original Magoi carp which also were black. A true black should have a nice dark sumi, almost as if the fish were dipped in an inkwell.
In Japan, the black koi symbolizes good luck, and you can almost always find a black koi hidden within a pond. If the fish has any white spots on it, or more of a Kumonryu than a pure black, and would represent life changes and transformations.
When selecting a black koi, be sure to look for a little older fish that has a nice dark sumi. Younger fish that present well with dark sumi have been known to add color as they age, adding in some bronze or red tones making them look rusted and no longer pure black. Although these fish can also be attractive, it can be a disappointment when one finds their black koi of 3 years has suddenly changed to a bronze color!