In the future it would be helpful to differentiate which sakai your talking about. I have over 20 years of experience with sakai-isawa and the Matsunosuke brand of koi. In his better koi, especially nisai and sanai, the black comes up and down before settling in. When a fish grows it also affects the condition of the coloration as things stretch to accomodate size.
My advice would be not to be concerned as long as the water and koi are both healthy. It also helps from an opinion standpoint to see the picture of a koi at tosai, which it often returns to sumi wise and to be able to see a picture of the entire koi so that an individual sumi area can be compared with what is going on elsewhere in the koi.
in my decades of involvement with koi keepers too many are ready to off a koi in the first coupla years only to discover later that the koi turned out well with those willing to be patient.....
