What is that dreaded black speck you ask?
No, it is not that little wood wedge used to level something, but it is possible it is a Shimi. Shimi translates to a freckle. It is a black spot that is smaller than the size of a scale, and is located under the mucus layer, but only in the superficial epithelial layer. Koi show judges consider it a demerit, thus, the 'dreaded black spot.' Genetics are considered the primary cause, and Koi with purer, long bloodlines are considered more stable. Poor water quality and hard water may also be contributors to shimis, making them more prevalent if the Koi's genetics make it susceptible .
A Shimi can be surgically removed by anesthetizing the Koi and scraping it off with a scalpel, but the Koi must be monitored for secondary infection, and removal can leave a scar. Shimis are basically cosmetic and cause no health concerns, but are esthetically not desirable, particularly in a show Koi. But, take note, if the shimi moves, it is likely a parasite such as argulus, aka fish lice, and you need to investigate and treat appropriately.