There are four blessings that make up Birkat HaMazon . The first three are generally viewed as of Biblical origin, but according to some, the fourth is a separate, Rabbinic blessing.
R Yitzḥak bar Shmuel bar Marta said another proof in the name of Rav: Know that the fourth blessing – HaTov veHaMetiv – “Who is good and does good,” is not required by Torah law, as one who recites it begins to recite it with: Blessed, but does not conclude reciting it with: Blessed. This is the formula in all comparable blessings, as it was taught in a baraita: All blessings, one begins to recite them with: Blessed, and concludes reciting them with: Blessed, except for blessings over fruit, blessings over mitzvot, a blessing that is juxtaposed to another blessing, and the final blessing after Shema. There are among these blessings those that one who recites it begins to recite it with: Blessed, but does not conclude reciting it with: Blessed; and there are among these blessings that one who recites it concludes reciting it with: Blessed, but does not begin reciting it with: Blessed. The blessing: “Who is good and does good,” one who recites it begins to recite it with: Blessed, but does not conclude reciting it with: Blessed. This proves by inference that it is an independent blessing.
Generally speaking, the blessings recited over food or before performance of a mitzva are short blessings with a single theme. They contain neither pleas nor requests. As such, it is sufficient to open with: Blessed. More lengthy, complex blessings, e.g., Kiddush on, which must include praise of God’s creation, or the blessings prior to reciting Shema, which include a petition on behalf of the Jewish people, must both open and close with words of praise to the Almighty. When a blessing that is juxtaposed to another blessing, however, creating a series of blessings that follow one another, e.g., the eighteen blessings of the Amida prayer and the blessings of Grace after Meals, the first blessing opens with “Blessed,” while the subsequent blessings do not. That is either because the opening of the first blessing in the series is considered as providing an opening for all of the blessings in the series (Rashi; Rashbam Pesaḥim 104b) or because the closing “Blessed” of the previous blessing serves as an opening for the blessing that follows (Tosafot).