Come on - there has got to be some seminarians who can tell me what a priest does/should say we he blesses you outside the liturgy.
Typically, if someone asks a priest for a blessing, the priest will say "O Kyrios" or "Tou Kyriou" which means "The Lord" or "Of the Lord." In both cases, the point is that the Lord blesses us, or that blessings come from the Lord.
If your priest is a bit more formal or one who avoids shorthand, he might say the whole thing: "O Kyrios evlogei" or "O Kyrios va sas evlogei," i.e. "The Lord blesses" or "May the Lord bless you."