Which blessings will we say when Moshiach comes?
Rabbi Shlomo Zalmen Auerbach ruled that when Moshiach comes we will say four blessings. There is a blessing that is said upon seeing an outstanding Torah scholar: "Blessed are You, the Lord our G-d, who has granted of His wisdom to those who fear Him." There is also a blessing said upon seeing a king: "Blessed are You... who has granted of His honor to those who fear Him."
There is also a blessing made when seeing over 600,000 people gathered in one place at once. The blessing expresses our awe at G-d who has created such a multitude of people, all different from another, yet understands the needs of each one. The blessing is "Blessed are You... master of secrets." Presumably, when Moshiach comes the entire Jewish nation will join in one place and we will recite the blessing.
A fourth blessing is the one we make upon hearing good tidings or experiencing extraordinary joy of any sort: "Blessed are You... who has given us life and sustained us and allowed us to reach this occasion." This blessing is said upon seeing a close friend with whom one has not had communication in over 30 days. Certainly, our joy upon seeing Moshiach will far surpass this joy and will warrant this blessing.
Rabbi Chaim Palgi mentions yet another blessing. “Apparently we will also recite the blessing of 'He who redeems Israel', thanking Him for redeeming us from this bitter exile.” This is patterned after the final blessing of the Hagaddah at the end of the narrative portion of the Pesach seder: “We praise You with a new song on our redemption and the release of our souls, and we bless 'He who redeems Israel.’”
(Lev Chaim II, chapter 42. Minchas Shlomo, 81,27. Toras Menachem 5749, vol. I p. 207. Ateres Paz, Orach Chaim I, notes, ch. 15)