Rashi on Leviticus 13:31/רש"י על ויקרא י״ג:ל״א

1ושער שחור אין בו AND THERE IS NO BLACK HAIR IN IT [THE PRIEST SHALL SHUT HIM UP] — Consequently, if there is black hair in it, he is clean, and does not require to be shut up — for black hair is a characteristic of cleanness in cases of נתקים, as it is stated, (v. 37) “if there is black hair grown up in it … [he is clean]” (Sifra, Tazria Parashat Nega'im, Chapter 8 9).