Daf Yomi - Beitzah 8 - A blessing for a completed mitzvah by Reb Jay

One cannot slaughter on Yom Tov a koy, which is a specie that the Chachamim were uncertain whether it is categorized as a beheimah, a domestic animal, or a chaya, a wild animal. If one slaughtered a koy, since it may be a beheimah, one cannot cover its blood. There is a concern that one who observes someone else covering the blood of a koy on Yom Tov will assume that a koy is a chaya, as we would not permit one to exert himself on Yom Tov for an uncertain specie and one would then permit the cheilev of a koy to be eaten, when in fact, because of its uncertain status, one is prohibited form eating the cheilev of a koy.

The Rambam writes that one who covers the blood of a beheimah that is kelayim, an animal that was bred from a chaya and a beheimah, one does not recite the blessing that is normally recited for the mitzvah of covering the blood. Rav Chaim Brisker wonders why one does not recite a blessing in such a case as there is reason to say that the animal that was slaughtered was a chaya which requires that its blood be covered. Rav Chaim answers that although there is reason to require that its blood be covered, the converse is also true, as there is reason to exempt one from covering the blood of this animal. The mitzvah is thus lacking a full requirement and for this reason one does not recite a blessing when covering the blood. Rav Chaim likens this ruling to a different ruling of the Rambam. The Rambam writes that when a child is born circumcised, or if a convert to Judaism was already circumcised prior to his conversion, we draw some blood, known as hatafas dam bris, but one does not recite a blessing on this procedure, although this is not a case of uncertainty. The reason for this ruling is because the mitzvah cannot be performed completely so one does not recite a blessing when performing such a mitzvah.

Rav Soloveitchek in Harerei Kedem likens this case to sitting in the Sukkah on Shemini Atzeres, as there is a requirement to sit in the Sukkah on Shemini Atzeres, yet there is also reason to exempt one from sitting in the Sukkah on Shemini Atzeres. Given the fact that one cannot fulfill the mitzvah of sitting in the Sukkah completely, he will not recite a blessing for sitting in the Sukkah.