Matthew Poole and continuators on Leviticus 13:31

31 And if the priest look on the plague of the scall, and behold, it be not in fight deeper than the skin, and that there is no black hair in it : then the priest shall shut up him that hath the plague of the scall, seven dayes.

For had that appeared, it had ended the doubt, the black hair being a sign of soundness and strength of nature, ver. 37. as the yellow hair was a sign of unsoundness.