We call her blessed because she blessed the world by saying “yes” to God: “Let it be with me according to your word.”
The blessing that God will work through her will come by way of the Son that she bears, God’s own Son, who will come to save his people from sin, but also to overturn the power structures that had so long held people in slavery. She sings:
His mercy is for those who fear him
from generation to generation.
He has shown strength with his arm;
he has scattered the proud in the thoughts of their hearts.
He has brought down the powerful from their thrones,
and lifted up the lowly;
he has filled the hungry with good things,
and sent the rich away empty.