The mother of Christ was specifically called "blessed" at least as far back as the end of the second century AD, and given the title "the Blessed Virgin" at least as far back as the early fifth century.
CCEL.org, the Christian Classics Ethereal Library, lists several references for "blessed Mary" from the Gospel of Pseudo-Matthew. For example:
And when he had thus said, the angel ordered the beast to stand, for the time when she should bring forth was at hand; and he commanded the blessed Mary to come down off the animal, and go into a recess under a cavern, in which there never was light, but always darkness, because the light of day could not reach it. And when the blessed Mary had gone into it, it began to shine with as much brightness as if it were the sixth hour of the day. The light from God so shone in the cave, that neither by day nor night was light wanting as long as the blessed Mary was there.
(Gospel of Pseudo-Matthew, Chapter 18; in Ante-Nicene Fathers, Volume 8: The Twelve Patriarchs, Excerpts and Epistles, The Clementina, Apocrypha, Decretals, Memoirs of Edessa and Syriac Documents, Remains of the First Ages: Philip Schaff, ed.)
Wikipedia quotes (but does not provide a reference to) a work dating this apocryphon to the early seventh century.
There's also a letter from Pope Gregory the Great (Gregory I), who was Pope from 590 to 604 AD:
To Peter, Subdeacon.
Gregory to Peter, &c.
Being exceedingly desirous of observing the festivals of saints, we have thought it needful to address this our letter of direction to thy Experience, informing thee that we have arranged for the dedication with all solemnity, with the help of the Lord, in the month of August, of the Oratory of the Blessed Mary lately built in the cell of brethren where the abbot Marinianus is known to preside, to the end that what we have begun may through the Lord’s operation be completed. But, inasmuch as the poverty of that cell requires that we should assist in that day of festival, we therefore desire thee to give for celebrating the dedication, to be distributed to the poor, ten solidi in gold, thirty amphoræ of wine, two hundred lambs, two orcæ of oil, twelve wethers, and a hundred hens, which may be afterwards charged in thy accounts. Provide therefore for this being done at once without any delay, that our desires, God granting it, may take speedy effect.
("Register of the Epistles of St. Gregory the Great", translated by James Barmby)
Even earlier than this, I see that this series of writings of the Ante-Nicene Fathers includes letters from Theodoret of Cyrus, who lived in the early fifth century. He writes (to a "Bishop Timotheus", who may be Pope Timothy II of Alexandria):
And, again, Gabriel says to the blessed Virgin, "Behold thou shalt conceive in thy womb, and bring forth a son and shalt call His name Jesus".
Earlier still, there is a fragment of a sermon by Saint Hippolytus:
Tell me, O blessed Mary, what that was that was conceived by thee in the womb, and what that was that was born by thee in thy virgin matrix.
This then pushes back use of "blessed" to describe the mother of Christ at least as far back as the late second or early third century AD.