John Gill Commentary Leviticus 13:31

"And if the priest look on the plague of the scall, and, behold, the appearance thereof be not deeper than the skin, and there be no black hair in it, then the priest shall shut up [him that hath] the plague of the scall seven days:" — Leviticus 13:31 (ASV)

And if the priest look on the plague of the scall, as it may appear in another person, brought to him for inspection and examination: and, behold, it is not in the sight deeper than the skin; it does not seem to be got into the flesh, or lower than the skin: and there is no black hair in it; or, "but no black hair in it";

For, as Jarchi says, if there was a black hair in it, he would be clean, and there would be no need of shutting up; for black hair in scalls is a sign of cleanness, as it is said, (Leviticus 13:37). It would be a clear case that such a man had no leprosy on him; for black hair is a token of a strong and healthful constitution; and there could remain no doubt about it, and it would require no further trial and examination. Ben Gersom says it means two black hairs; and further observes, that black hair in the midst of a scall is a sign of cleanness; but this being wanting,

then the priest shall shut up him who has the plague of the scall seven days; from the time of his viewing the scall; and so Ben Gersom, this is the seventh day from the time of looking upon the scall.