I’m thinking that this might be the Divine Praises:
Blessed be God
Blessed be His Holy Name
Blessed be Jesus Christ, True God and True Man
Blessed be His Holy Name
Blessed be His Most Sacred Heart
Blessed be His Most Precious Blood
Blessed be Jesus in the Most Holy Sacrament of the Altar,
etc.
(Google the rest of it.)
I love this prayer, and it was one of the first prayers that I memorized when I converted to Catholicism from Evangelical Protestantism.
I’m not sure why it is being prayed during the Mass, though. We pray it during the Benediction of the Blessed Sacrament, but never during Mass.
Surely this isn’t the “Blessed be” that the priest prays before the consecration. The congregation only answers, “Blessed be God forever,” and that’s not exactly something that has to be memorized. You hear it once, you’ve got it.