Sforno on Leviticus 13:30/ספורנו על ויקרא י״ג:ל׳

1נתק, a skin affliction in a part of the skin normally covered with hair is called a נתק. The word implies that the affliction uproots the hair follicles with the hair. This hair was not pulled out by human hands or by some kind of chemical or medication. The fact that some black hair remains in the skin proves that notwithstanding the other symptoms suggesting that the person suffering the affliction is ritually impure, the black hair’s presence saves him from such a determination by the priest.