Megillah, Chapter Three, Mishnah 7. This mishnah too continues to deal with the laws of the priestly blessing.
The text of the mishnah:
"Kohen sheyesh b'yadav mumin lo yisa es kapav" - a kohen who has blemishes on his hands does not raise his hands to bless the people.
Why not? There are two reasons for this:
Because the people look at his hands - and there is a problem with looking at the hands of the kohanim, for at least in the Beis HaMikdash the Divine Presence rested upon their hands, and from there comes the instruction not to look.
Because of distraction - the people will be distracted and will not pay attention to the berachah, whereas it is incumbent upon them to direct their attention to the blessing of the kohanim.
Rabbi Yehudah's addition:
"V'Rabbi Yehudah omer: af mi shehayu yadav tzevuos istis u'fuah lo yisa es kapav, mipnei sheha'am mistaklin bo".
Istis - a type of blue dye.
Puah - a type of red dye.
A kohen whose hands are stained with these dyes does not go up to the platform, for the reason that the people look at him.
Two qualifications to this law:
All of this assumes that the kohanim do not cover themselves with a tallis.
And likewise, that the people are not accustomed to such a sight. But if most of the people of that community work with these dyes and their hands are stained, and people are used to it, it does not stand out and there is no concern.
In summary: in this mishnah we learned that a kohen who has blemishes on his hands does not raise his hands to bless the people, because the people look at his hands and because of distraction from the berachah; and according to Rabbi Yehudah, even a kohen whose hands are stained with istis and puah does not raise his hands, because the people look at him - and all of this applies when his hands are not covered with a tallis and when people are not accustomed to such a sight.