In the Hail Mary, we say, “Blessed is the fruit of your womb.” Jesus was holy from the very beginning. How can he be blessed by anyone?
That part of the prayer quotes Elizabeth’s greeting to Mary (Lk 1:42b). Elizabeth is not giving a blessing but simply acknowledging a blessing already given by God. If you sneeze and I say, “God bless you,” I am not the source of that blessing but am simply recognizing that you have already been blessed by God.