The Karasugoi or Karasu is an all black Koi. Karasu means "crow' They can and will however have orange, red, white or yellow bellies in some cases that is not readily visible, especially when Koi are viewed from above. They are said to be good luck in Japan and everyone should have one in their ponds. So there is not a lot to get into with this breed. They are true black Koi, and come in Doitsu (scaless or partially scaled) or Wagoi (fully scaled) versions.
Understand however that Karasu as with many other varieties of Koi can change colors based on the background color of the system they are housed in. In other words if you put certain breeds in a white tank, certain colors of that breed will fade in an attempt to camouflage themselves. Yes, this is similar to what chameleons do. With Koi however it is not as intense or accurate a change. They simply lighten in a light system, and darken in a dark system. a True Karasu however will not lighten much if at all, and remain black. I have seen many folks refer to some fish as Karasu when in reality they are very dark brown Koi. These Koi will lighten when put into a lighter background like a white, blue, or pale color system. many folks do not realize that many breeds of Koi have this ability.
A Good Karasu
It's simple....you want them as black as hell! Body confirmation and scale reticulation in Wagoi version is also critical as with all breeds of Koi.