"Why do the Orthodox call May the "Mother of God" (theotokos in Greek, "Bogoroditsta" in Slavonic)? This term seem to my ears to be at least extreme and possibly a blasphemy."
The term "Theotokos" is of long-standing, in fact it originated before Christ was born!
Recall that after Mary heard the annunciation of her pregnanct by a "seedless conception" form the Archangerl Gabriel, she made haste to go into the Hill country to be with her kins-woman Elizabeth, who was aslo with child, about 6 months further along in her pregnancy. As Mary enters the room, The Holy Apostle Luke tells us:
"And it came to pass, that, when Elisabeth heard the salutation of Mary, the babe leaped in her womb; and Elisabeth was filled with the Holy Ghost: {42} And she spake out with a loud voice, and said, Blessed art thou among women, and blessed is the fruit of thy womb. {43} And whence is this to me, that the mother of my Lord should come to me? {44} For, lo, as soon as the voice of thy salutation sounded in mine ears, the babe leaped in my womb for joy. " (Luke 1:41-43)
Elizabeth uses the same language that means "Mother of God" The term Lord in the New Testament is the same for the OT. The term is "Kuriou" in Greek. This is the term the LXX uses to translate Yehovah (YHWH) (Jehovah) in the Old Testament. The English translation of the term is Generally translated "Lord." So for Elizabeth to call Mary "mother of my Lord" is equivalent to calling Mary the "Mother of God."
Afanasy Bailey