The Torah teaches: “If a person will have on the skin of his flesh a s’eis or a sapachas, or a baheres, and it will become a Tzoraas affliction on the skin of his flesh; he shall be brought to Aharon the Kohen, or to one of his sons the Kohanim” (Vayikra 13:2). Rashi states: “It is a decree of Scripture that there is neither impurity of affliction of Tzora’as nor their purification except by word of a Kohen.” In other words, even if the Kohen is not an expert in these laws, and even if he does not know what he is talking about (the laws of Tzoraas are indeed complicated and complex), if the Kohen is told by a Talmid Chochom what to say and he parrots the words of the Talmid Chochom, the blemish is pronounced tahor [pure] or tamei [impure] based on the proclamation of the Kohen, not on the proclamation of the Talmid Chochom who is not a Kohen.
The Rambam elaborates on this principle in Hilchos Tumas Tzoraas (9:2): “Even though anyone is fit to inspect blemishes, purity and impurity (tumah and tahara) depends on the Kohen. How so? A Kohen who does not know what to look for has a Talmid Chochom inspect (the blemish) and has the Talmid Chochom instruct him ‘Say it is impure’ and the Kohen says ‘impure’; or the Talmid Chochom instructs ‘Say it is pure’ and the Kohen says ‘pure’; or the Talmid Chochom instructs ‘Put him in a suspended state for another week’ and the Kohen puts him in a state of suspension (masgeero) as it is written ‘based on their mouths shall be every fight and every blemish’ (Devorim 21:5). And even if the Kohen is a minor (katan) or an imbecile (shoteh), the Talmid Chochom instructs him and he decides whether the person is definitely impure, pure, or suspended further...”
Based on this Rambam, the Minchas Chinuch speculates whether or not the proclamation (based on direction of a Talmid Chochom) of a blind Kohen regarding a nega would be effective. The Minchas Chinuch discusses the possibility. However, it is implicit in the Meiri and also from Rashi and Tosfos in Sanhedrin 34 as well, that a blind Kohen CANNOT rule on the status of Tzoraas, even if so directed by a Talmid Chochom. This is learned out from the expression (Vayikra 13:12) “l’chol mar’eh einei haKohen” (wherever the eyes of the Kohen can see). Rashi on this pasuk quotes the Toras Kohanim, Negaim, perek 4:4: “To the exclusion of a Kohen whose power of vision is impaired.” Under normal circumstances, it would be obvious to us that a blind Kohen cannot rule on such matters of visual determination. However, in light of the earlier cited Rambam that even a minor or mentally deficient Kohen can rule based on the guidance of a Talmid Chochom, the exclusion of a blind Kohen is somewhat of a novelty.
