Talmud, Bava Metzia 86a
“It was debated in the Academy of Heaven: If the white patch precedes the white hair, it is impure; if the white hair precedes the white patch, it is pure; but what if there is doubt (as to which came first)?

The Holy One, Blessed be He, said: It is pure.

The entire Academy of Heaven said: It is impure.

Said they: Who shall decide it for us? Rabbah bar Nachmeini. For Rabbah bar Nachmeini had declared: I am singular[ly knowledgeable] in the laws of tzaraat… They dispatched a messenger [to bring him to heaven]… Said [Rabbah]: Tahor! Tahor! (Pure, pure).”

Had you looked closely the issue here that is being portrayed is in regard to one persons view verses the majority and not in regard to pure verses inpure as you assumed. Its issue is meant to mock Yeshua the Messiah here in the Talmud passage. When investigating the issue in order to understand the laws of doubt if there is no sign in which is visible which can prove that its possible the patch preceeded the white hair then the thing is deemed pure. The same goes for a Niddah who bleeds if the blood can not be proven to have come from the cervix nor the area which niddah blood is found yet we find blood the doubt is declaired pure. However, if there is signs of possibility to its coming from the area such as a blood spot on the inner thigh this sign causes the doubt to be regarded as impure. When we look closer as the euphamism of Rabbah Bar Nachmeini who declairs things himself as a singular person who declaired the laws of Torah if we look at the parts of the passages which are not quoted here we learn the law always follows the majority and therefore this man who is the code in the Talmud for Yeshua is wrong and we (meaning the sages of the Talmud) do not listen to his words but rather we follow the majority.

Acts 10:9-19
9 About noon the following day as they were on their journey and approaching the city, Peter went up on the roof to pray.
10 He became hungry and wanted something to eat, and while the meal was being prepared, he fell into a trance.
11 He saw heaven opened and something like a large sheet being let down to earth by its four corners. ”
12 It contained all kinds of four-footed animals, as well as reptiles of the earth and birds of the air.
13 Then a voice told him, “Get up, Peter. Kill and eat.”
14 “Surely not, Lord!” Peter replied. “I have never eaten anything impure or unclean.”
15 The voice spoke to him a second time, “Do not call anything impure that G-d has made clean.”

The whole point we find here, as is alluded to in the text, is that Even (Peter) saw these seemingly unclean animals before him being presented to him as food. From the onslot Even rejects them becuase of his outward view, in which he is judgeing, and therefore he boldly proclaims, “Not so Lord I have never eaten thats which is impure or unclean.” But what needs to be asked is why would G-d command him to eat it? The answer is unfortunately more obvious then we seemingly see, and because of its obvious meaning what is unfortunate is the simple answer unfortunately becomes the less obvious responce to it. And what is it? The answer is, is anything which comes from heaven unclean? No! Rather, only the animals which come from the below have only this ability, therefore G-d is telling Even we can not judge the outside of something, “for look I presented to you that which outwardly seems unclean but to where did this thing come from, is it not I who brought it down from heaven to you and said eat?” This picture which was given three times represented the three gentiles who later Even declaired what was the meaning of the revelation G-d was seeking to teach him. That being we can not call a gentile unclean for many who are unknown to us Jews have turned to G-d and have been made pure by G-d who has presented these such ones to us whom G-d himself has made pure.