R. Akiva teaches his talmidim a drasha דם יהיה זובה בבשרה - דם ולא כתם, only a flow of blood is metamei but not a stain. Based on this R. Akiva rules that so long as something exists to attribute the stain to, even an injury that is presently scabbed and not bleeding, she would be tahora. The Sidrei Tahara (end of 190) explains that according to the Rambam a woman is only tamei d'roaysa if she is confident that the blood is from her, AND that she has a hargasha. The absence of either, such as the blood definitely being from her but coming without a hargasha, or the possibility of a hargasha but having something else to attribute the blood to i.e. finding blood after tashmish when she has some type of wound, she is tahora. The source for the requirement of hargasha is the drasha of Shmuel on 57b בבשרה - עד שתרגיש בבשרה, and the source of her knowing for sure that the blood is from her is from R. Akiva's drasha in our gemara. Both are true and accurate.
The Maharatz Chiyus asks from our Mishna on the Taz who writes (Y.D. 116) that any leniency that is explicit in the Torah, the Rabbonon don't have the authority to forbid. How then can they be go'zer on kesamim after the Torah says דם from which chazal understand is to the exclusion of a kesem? Although one can say simply that the inference of ולא כתם is merely an inference and wouldn't qualify as explicit in the Torah, the Maharatz Chiyus cites another Taz (O.C. 588) who says that chazal couldn't prohibit doing a Bris on Shabbos out of fear that one may carry as they did by shofar and lulav because it is explicit in the Torah that a Bris can be done on Shabbos. The Torah says וביום השמיני ימול את בשר ערלתו and chazal understand that ביום implies even shabbos. Just as this is considered "explicit" in the Torah since that is how chazal interpret the word of וביום, so too we should consider the exclusion of a kesem from Nida to be explicit in the Torah. How then can chazal be go'zer on kesamim?
It seems to me that there is a clear distinction between considering it explicit in the Torah that Bris can be done on shabbos, and considering the heter of kesamim explicit in the the Torah. When the Torah writes the word וביום, chazal teach us that it means as it sounds, every day of the week including shabbos. Therefore, the Torah is explicitly saying according to the translation of chazal that one can do a bris on shabbos. That is why the Taz holds the Rabbonon don't have the authority to forbid it. However, in the context of a stain, although chazal understand the term דם to include only clear bleeding from her body, not a stain, the Torah doesn't make a statement about what is not tamei, it only makes a statement about what is tamei. The Torah says that "dam" is tamei, which chazal interpret to refer to a flow, but the Torah never says explicitly says that anything aside from that it is tahor. It is a correct inference, therefore kesamim are not tamei on a Torah level, but it doesn't qualify as explicit to prevent chazal from prohibiting it.