TT #199 · Parshas Tazria · Aliyah 5 · Passuk 31
English: If the priest examines the netek and its appearance is not deeper than the skin, and there is no black hair in it, he shall isolate the netek for seven days.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:31
Torah Temimah cites Sifra (Toras Kohanim) for the special protective role of black hair as distinguished from yellow hair.
Sifra — Vayikra 13, Midrash Sefer
English: Why does the verse specify black hair? Black hair protects the person from the netek impurity ruling; yellow hair does not provide that protection.
Textual derivation
For a netek, black hair can prevent the impurity ruling; yellow hair does not provide that protection. This is a specific rule for the netek signs, not a general claim about all skin-affliction appearances.