In the Book of Blessings approved for the USA in 1989 there is a ceremony: Chapter 48 Order for the Blessing of a Christmas Manger or Nativity Scene.
To my knowledge, this is only approved for the USA. It is not in the original Latin edition of the Book of Blessings.
The Introduction includes:
“1542 The blessing of the Christmas manger or nativity scene, according to pastoral circumstances, may take place on the Vigil of Christmas or at another more suitable time.
1543 The blessing may be given during a celebration of the word of God, during Mass, or even during another service, e.g., a carol service.”
For the Order of Blessing Within Mass, it has instructions to explain the meaning of the celebration in the homily. Then it has:
“1563 The general intercessions follow, either in the form usual at Mass or in the form provided here. The celebrant concludes the intercessions with the prayer of blessing. …”.
So this is the only time the blessing may take place in Mass, at the end of the Prayer of the Faithful.