Upon returning to Ammonihah, Alma was met by Amulek, a citizen of the city, who had been advised by an angel to welcome "a prophet." Amulek "received him into his house … and he brought forth bread and meat and set before Alma" (8:21). After eating the meal set before him, Alma "blessed Amulek and his house, and he gave thanks unto God" (8:22). Whereas generally, Latter-day Saints and other Christians are accustomed to offering a blessing before meals, "In Judaism, while a brief blessing is recited before eating, a series of longer blessings … follows the meal." It is "a central feature of the liturgical service in the Jewish home" (see KnoWhy #115).
In 1997, Angela M. Crowell and John A. Tvedtnes explained, "Four blessings come after the consumption of bread, while separate blessings are offered for other foods, depending on their nature and origin." Deuteronomy 8:10, provided the basis for this practice: "When thou hast eaten and art full, then thou shalt bless the Lord thy God for the good land which he hath given thee." Amulek had apparently fed and filled Alma, and so Alma pronounced blessings as expected.
The practice is also found in the Mishnah and among the Dead Sea Scrolls. The Jewish book of Jubilees (ca. 161–140 BC) depicts the patriarchs engaging in this practice (Jubilees 22:4–9). One story in Mishnah Sukkah 2:5 records, "When Rabbi Zadok ate only a small portion of food, he did not say the blessing afterward." This was probably because Deuteronomy 8:10 calls for a blessing only if one has eaten and is full.
Alma’s blessing anticipated the time when the Lord would bestow a blessing after a messianic meal. The Savior followed a similar pattern during his first day among the Nephites at the temple in Bountiful. As the sacrament was administered to the people there in 3 Nephi 18, they were "filled" (v. 9), after which Jesus "blessed" them (3 Nephi 18:10, 14).
Book of Mormon Central, "Why Did Alma Bless and Thank God After Eating?" KnoWhy 115 (June 6, 2016).
Crowell, Angela M., and John A. Tvedtnes, "Notes and Communications: The Nephite and Jewish Practice of Blessing God after Eating One’s Fill," Journal of Book of Mormon Studies 6, no. 2 (1997): 251–254, quote on 252.
John W. Welch, "From Presence to Practice: Jesus, the Sacrament Prayers, the Priesthood, and Church Discipline in 3 Nephi 18 and Moroni 2–6," Journal of Book of Mormon Studies 5, no. 1 (1996): 119–139.