When for some reason the original skin of the tuber is rubbed off and a new corky layer form, it is usual for it to be thicker and darker than the first one.
The damage is regarded as a minor defect because of the contrast in colour between the original and secondary skins. In conjunction with the damage to the original skin there is a corresponding loss of fluid and the underlying tissue becomes soft. This is particularly noticeable in the case of roughly handled new potatoes.