TT #213 · Parshas Tazria · Aliyah 5 · Passuk 37
English: If the netek remains unchanged and black hair has grown in it, the netek is healed; he is clean, and the priest shall pronounce him clean.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:37
Torah Temimah cites Sifra (Toras Kohanim) for black hair present from the beginning or growing later.
Sifra — Vayikra 13, Midrash Sefer
English: The wording includes both black hair that grows later and black hair that was present from the beginning.
Textual derivation
Black hair is included whether it was present at the start or grew later in the course of the netek.