Check it out.

I'm not saying that the blood cannot be from her, I'm just asking where we find that a person who examined something, the Sages say, "Perhaps you didn't examine it well and not properly." Furthermore, the Gemara later brings a ruling regarding a woman who examined her robe and lent it to her friend and found blood on the robe, which the first one is completely pure and the second is impure according to the rabbis. I'm asking because according to the method of Toss, the first one is not impure according to the Torah, since she didn't examine it well, but it was permissible for the Sages to impure it according to the rabbis because it is something that becomes impure. But perhaps it can be said that when a person says that he tested, he is completely trusted, especially since it is explained there in the Gemara that it is easier to test soil than clothing, and there is another Gemara, page 14, where Samuel defiles from the Torah a woman who finds blood on her thigh that has passed to her because the one who tested her, and specifically tested her, and defiles from the Torah, even though it is possible to say that the test came out clean and the blood on the thigh came from another place, as explained there by Rashi.

As stated, the lack of feeling is a ri'utah. Beyond that, it is a stain and not a real sight, and stains have different meanings (such as food matters) unlike the laws of the Torah. In particular, they did not rule out stains if they were found on something that does not accept impurity (and land does not accept impurity).

The Rabbi forgives, but that does not answer the question. After all, it is clear there in the words of Rav Ashi that he was obliged to place the words of Shmuel on the ground in order to purify it completely, that if she sat on a cloak, she would be impure (from the Rabbis), and according to what the Rabbi says, why does Rav Ashi not place her on a cloak as well and place her on the presumption of purity? And if the Rabbi answers that they were strict and decreed that it was precisely that she tested the ground before sitting, that if the ground was not tested, then the fact that she tested the ground only detracts from the fact that it is more certain that it came from her body, and therefore Rav Ashi was obliged to place her on the ground. And what the Rabbi says is that it is a stain and not evidence, what does he mean? The fact that she did not feel it does not mean that it is certainly not evidence. Perhaps it is, after all, according to the Toss’ method, feeling is a sign, not a reason for da’reita, and it can be a reality that will be impure from the Torah even without evidence and less than gnawing on the ground in a way that it is certainly from her body. And according to what the Rabbi says, there is no difference between a thing that is tested and not tested. In a thing that is tested, one can base a presumption on it. And after all, in some issues the law changes depending on whether or not it is tested.
Thank you very much for your patience.

I am not familiar with the issue, and it is difficult for me to answer in detail. I will just say that I have included several branches here. There is both a presumption and a ruling that something does not receive impurity, and also a concern that the test is incorrect. You assume that each of them individually is sufficient, but that is not necessarily the case. It is possible that all of them are needed. So put them in the ground so that there is something that does not receive impurity, and only then will the other considerations come into play.