Menachos 21a: One who eats cooked blood does not transgress [the Torah prohibition].
מנחות כא ע”א: אמר זעירי א״ר חנינא: דם שבישלו אינו עובר עליו.
A woman came to the Taz and asked, “I cooked meat, but forgot whether I salted it beforehand.” The Taz permitted it because of ספק דרבנן לקולא – in a doubtful situation on an enactment of the Sages, we rule leniently. We salt meat to remove its blood, but even if we did not remove the blood, once the meat is cooked, cooked blood is forbidden only Rabbinically. Therefore, salting is only a Rabbinic question. Although the rule is that even a doubtful situation of a Rabbinic law, where there is a חזקה – a status quo situation where it is forbidden – we rule strictly, in this case the fact that the meat originally had blood is not considered a חזקה.
The Taz proves this from a case in Shulchan Aruch: meat was known to be soaked and salted, and then a non-Jewish employee cooked it, saying that he soaked it a second time. We rely on his words if he is מסיח לפי תומו – he is speaking casually, not aware that we are relying on him. Now, “speaking casually” is only relied upon when the prohibition is Rabbinic and there is no חזקה. So it must be the fact that the meat originally had blood is not considered a חזקה.
Why not? The Chavos Daas explains that the meat is one thing and its blood is another. The meat itself was never forbidden, only it had blood mixed into it. By salting, we are separating the two, but we are not permitting anything that was previously forbidden.
The Shach (Nekudos Hakesef) disagrees and argues that in the case of the non-Jew, if there were no חזקה we would permit the meat even without the non-Jew’s casual testimony because every Rabbinic doubt is permitted (ספק דרבנן לקולא). From the fact that we do require his מסיח לפי תומו, we see that blood in the meat is called איתחזק איסורא. And why then is he believed? The Shach (Sifsei Kohein) already explains that non-Jews care about cleanliness and it is likely that he washed the meat anyway. Therefore, here in the case of the woman who is unsure whether she salted the meat, we follow the חזקה and the meat is forbidden.
The Taz adds that she probably followed habit and salted the meat. To this, the Shach responds that since salting is a big job, the fact that she doesn’t remember it probably means she didn’t do it
[Seemingly there is a big question on this Taz. If blood in meat is not איתחזק איסורא, then treife taste absorbed in a pot is also not איזחזק איסורא for the same reason: the pot is one thing and the treife taste is another, and we are only separating the two. But the whole source for the rule that מסיח לפי תומו is not relied upon by an איסורא דרבנן דאיתחזק איסורא is the end of Siman 137, which says that aנכרי מסיח לפי תומו is not believed to say that a pot was kashered!
This same question can be asked in Siman 102 in the laws of דבר שיש לו מתירין. The Rema (s’if 4) states that meat with blood in it is not דבר שיש לו מתירין because the meat itself doesn’t need to become permitted; the blood merely has to be extracted. But in s’if 3 all agree that treif utensils would be דבר שיש לו מתירין, if not for the fact that it would be expensive to kasher them.]
Source: Taz, Yoreh Deah 69:24