Blessings of the Shema
There are three general types of blessings: 1) Blessings said for mitzvot, like blowing shofar and putting on tefillin; 2) Blessings recited on things that one derives pleasure from, like food and drinks; 3) Blessings of praise and gratitude, like ‘Baruch she’amar’ and shehechyanu.
What type of blessings are the “Blessings of the Shema”? From the name of these blessings and the fact that we say them together with both the evening and morning Shema it would seem that they are like the first type of blessings, blessings on a mitzvah. Yet, if they are blessings instituted for the mitzvah of Shema, why don't we say the usual phrasing: “Who has sanctified us through His commandments, and has commanded us to...” as is found in other blessings for mitzvot?
The Shulchan Aruch writes: If one says the Shema without reciting its blessings he still fulfills the mitzvah of Shema, but he must still go back and read the blessings of the Shema without reciting the Shema (Laws of Shema 60:2). The Mishneh Berurah explains this ruling as follows: One fulfills his obligation of saying Shema even if he does not say the blessings of Shema at all. However, he has not fulfilled his obligation to recite the blessings of the Shema. He may say the blessings without saying the Shema since these blessings were not instituted specifically for the mitzvah of the Shema, as we see from the text of the blessings that does not include the phrase “Who has sanctified us with His mitzvot....”
From the ruling of the Shulchan Aruch and explanation of the Mishneh Berurah it is clear that the blessings of the Shema are not said for the mitzvah of the Shema. This is also proven from the fact that one can still recite the blessings of the Shema for one full hour after the time of the Shema has already passed, and according to some authorities he may recite them during the entire day (Rambam, Pri Chadash, Chida).
They are, in fact, blessings of praise and gratitude. That is why even if one already fulfilled the mitzvah of the Shema he still needs to say these blessings. Regarding a blessing said for a mitzvah it is forbidden to recite a blessing after one already fulfilled the mitzvah.
It must be noted that the Shema may be said more than several times a day, just as one may read any passage from the Torah numerous times. Accordingly, the Shulchan Aruch writes that one should say the Shema when saying the blessings of the Shema even if he has already fulfilled the mitzvah of the Shema.