כה תברכו את בני ישראל
“Thus you should bless the bnei Israel”
In Numbers, Chapter 6, verse 23, the Torah Temimah mentions the Gemora Taanit 26b that asks why is the chapter of the Cohanim blessing the Children of Israel written right next to the chapter explaining the laws of Nazir [one who takes vows to be holy unto God]? To teach that just like a Nazir is forbidden from wine, so too a Cohen is forbidden to do the temple service while drunk.
The gemora there expands this teaching to include that a Cohen who will performs the congregational blessing during (synagogue) prayers must also not be drunk. Further the gemora in the same section asks whether a Cohen with a defect (a ‘baal mum’) can also perform the blessings (duchan). The gemora concludes that a Nazir who is a baal mum is accepted, so therefore a Cohen who is a baal mum must also be allowed to perform the blessings during the synagogue service. Tosafot in the same section also agrees.
The Torah Temimah then goes on to say that he is astounded both at the gemora and at Tosafot for even having a question as to whether a Cohen baal mum could duchan or not. Behold, he says, there is a Sifri in Parshas Shoftim that deduces from a verse in Devarim 21,5 “and the Cohanim, the sons of Levi” came to do the service that the extra phrase “sons of Levi” (of course they were the sons of Levi!) teaches that even Cohanim who are baal mum can perform the blessing. If the Sifri concludes that they can perform the blessing, why is the gemora even discussing it??!! [The Sifri was written during the period of the Taanaim which pre-dates the Gemora.]
And furthermore, the Torah Temimah continues, I am astounded that a Gemora Megila 24b and a Mishna there conclude that a Cohen baal mum cannot perform the blessing because the people will stare at his hands (because there are curious about his defects). Later commentaries then conclude that in our day, a Cohen with a blemish should not do the priestly blessing during the synagogue service. The Torah Temimah says that he cannot understand why rabbis are deducing laws to forbid something that is already allowed.