(כה) וְאִשָּׁ֡ה כִּֽי־יָזוּב֩ ז֨וֹב דָּמָ֜הּ יָמִ֣ים רַבִּ֗ים בְּלֹא֙ עֶת־נִדָּתָ֔הּ א֥וֹ כִֽי־תָז֖וּב עַל־נִדָּתָ֑הּ כָּל־יְמֵ֞י ז֣וֹב טֻמְאָתָ֗הּ כִּימֵ֧י נִדָּתָ֛הּ תִּהְיֶ֖ה טְמֵאָ֥ה הִֽוא׃... (כח) וְאִֽם־טָהֲרָ֖ה מִזּוֹבָ֑הּ וְסָ֥פְרָה לָּ֛הּ שִׁבְעַ֥ת יָמִ֖ים וְאַחַ֥ר תִּטְהָֽר׃
(25) When a woman has had a discharge of blood for many days, not at the time of her impurity, or when she has a discharge beyond her period of impurity, she shall be unclean, as though at the time of her impurity, as long as her discharge lasts...(28) When she becomes clean of her discharge, she shall count off seven days, and after that she shall be clean.
§ The mishna stated: Or she engages in sexual intercourse with him while she has the status of a menstruating woman. The Gemara asks: What are the circumstances? If he knows about her that she is a menstruating woman, he should abstain. And if he does not know, then he should rely on her. Because Rav Ḥinnana bar Kahana said that Shmuel said: From where is it derived that a menstruating woman can count the days for herself, and that she is trusted to testify that she did so? As it is stated: “Then she shall count to herself seven days” (Leviticus 15:28). “To herself” means by herself, and she may be trusted that she did so.
She should count for herself: If you were to ask why she doesn't need to say a blessing on this counting like we say when counting the Omer, which uses the same Hebrew word "vesafra", the answer is that we only say the blessing on a count which we know we will be able to complete in order, but in this case in which the count would be interrupted if she say blood we do not make the blessing.
I looked carefully at the words of the Tosafot, who said that she shouldn't say a blessing because it might end up being a "bracha levatalah" because the count won't be completed, but there is an obligation to count without a bracha, because of the language "vesafra lah". And we can't say that the fact that you don't say a bracha means there isn't an obligation to verbally count...And my custom all my days has been to tell my wife to count in front of me just before sunset, "Today is the x day of the days of whiteness" until we reach day 7.
In the Tosafot about the woman counting for herself, the fundamental question seems to be why the Talmud assumes that she is only counting for herself, because without it saying this