Kesem or tzeva?

The Mishna in nida 62a says that if you wash a garment with a stain with the 7 special cleansing agents and it doesn't come out then you can assume it's a tzeva and therefore the teharos remain tehoros. Tosafos quotes Rashi in Sandhedrin on daf 49b who says that it's a vadai tzeva. Because these cleansing agents didn't take the stain out we know for sure that it can't be blood. Rashi in our gemara is mashma like that also in a couple of Rashis (one in the Mishna and one on the top of amud beis). However, we were bothered this morning by one Rashi on amud beis (d"h taharosav tehoros) which sounds like he is learning like Tosafos. I saw now that the Yosef Daas gemara points that out also. Rashi says that even though you did the test it still might be dam. I was thinking (and it actually could be R' Tendler said this morning but I'm not positive if this is what he meant) that it could be that the Rashi there is going in shitas R' Chiya. R' Chiya holds that even though you know for sure it is dam then it is still tahor if you do the test and it doesn't come out. According to that you can't say that it's definitely not dam because we know it is dam. However the other Rashis are all talking about a kesem like our Mishna is talking about. Rashi's saying that in those shitos the 7 cleansing agents would almost definitely take it out so therefore if it doesn't you can be certain that it's not blood.

Tosafos asks a series of questions on Rashi and doesn't like his pshat (the Rashash tries to answer some of them). Tosafos says that when the Mishna says that it's a tzeva if it doesn't come out with the cleansing agents, it doens't mean that it's definitely a tzeva - it just means that it's likely or it might be a tzeva and therefore we can be meikil.

There are a number of nafka mina between their shitos but one major one is discussed by the Rishonim. In the case of the Mishna where we said that it is presumed tahor because it didn't come out with the cleansing agents, is she muteres to her husband also? The Rambam paskens that she is because this test proves that this isn't dam. So even though there is nothing to be tole this big stain on we still assume it's not dam because it didn't come out. It sounds like he's learning our gemara like Rashi. The Raavad others vehemently disagree and say that she is temeia. One raaya they bring is from our Mishna. The case was if you took the garment to the mikva and then you did the test then we say that we can consider it tahor. It implies that if you didn't take it to the mikva then even the garment would be tamei even if the cleansing agents couldn't get it off. So you see that the cleansing agents don't prove that it wasn't dam. Just that together with the fact that you took it to the mikva you can assume that it's already batel and therefore won't be metamei anymore.

For more on this topic look at the nosei keilim on the Rambam in hilchus isurei bia (9:38). Also the Ritva and Ran and Aruch Laner (and I'm sure many others) discuss the various shitos in the gemara.