Tosfos (Kesubos 72a) asks why a women who counts 7 neki’im until going to mikvah does not recite a bracha before each day’s count just as we recite a bracha before each day’s count of sefiras ha’omer. Tosfos answers that since it is possible that a woman might see dam during that week, forcing her to restart her count from the beginning, therefore “ain lah limnos”. The concluding line of Tosfos is every strange – the question was an issue of hilchos brachos, and we would have anticipated the answer to be that since there is the possibility of losing the count a bracha should not be recited. Tosfos concluding line implies that the possibility of being forced to restart means there is no mitzvah at all to count! The gilyon on the side of the page notes that the Shla”h corrected the girsa to read “ain lah levareich” – she should count, but not say a bracha. Based on this, some woman do have the chumra of verbally counting each day of the seven neki’im just as if counting sefiras ha’omer, albeit without a bracha. Many, however, do not articulate the count, in keeping with our girsa in Tosfos.
There is a gezeirah derabbanan that even after a woman finishes her count of 7 days, which happens the morning of the 7th day (since miktzas hayom k’kulo, part of a day counts as a day, the count is done at the start of the day), she does not go to mikvah until nightfall. Chazal were concerned for the off-chance that a woman would go to mikvah during the day and have relations with her husband and than discover dam before nightfall, rendering her count of the 7th day (and the preceding days) void. The exception to the normal rule is the case of a kallah. Since she is not yet married, there is no concern of the kallah being with her husband until after the wedding, therefore it is permissible for her to go to mikvah during the day (see meforshim on Y.D. 197:4 as I have ignored some details for the sake of simplicity).
The Badei haShulchan (based on an Avnei Nezer) asks: if a woman does not say a bracha on counting 7 neki'im lest she end up seeing dam and having to restart her count, why does the kallah say a bracha when she goes to mikvah on the 7th day before nightfall? There clearly is still a concern of seeing dam, as that is why we do not allow a regular woman to go to mikvah during the day. Why then are we not concerned lest the kallah see dam, rendering her bracha on tevila a bracha l'vatalah? What is the difference between these cases???