"Blessed Virgin" - not Saint? Probably the most common Catholic title for Mary, mother of Jesus, is "The Blessed Virgin" or "The Blessed Virgin Mary". But "Blessed", in Catholic hagiology, is typically used to denote people who are not saints yet:


  Blessed is the title for someone who has been beatified. ... an earlier step in the process toward sainthood.


(US Conference of Catholic Bishops, How To Cover The Catholic Church)

Catholics consider Mary the holiest of all human beings: 


  By her complete adherence to the Father’s will, to his Son’s redemptive work, and to every prompting of the Holy Spirit, the Virgin Mary is the Church’s model of faith and charity. Thus she is a "preeminent and... wholly unique member of the Church"; indeed, she is the "exemplary realization" (typus) of the Church. 


(Catechism of the Catholic Church, paragraph 967, quoting the dogmatic constitution Lumen Gentium, paragraphs 53 and 63)

This being the case, why is it that Mary is not&mdash;or at least not usually&mdash;given the (apparently highly appropriate) title "St. Mary"?
To add to the other answers given here, in the Catholic liturgy (Mass, Liturgy of the Hours), it is common to address the saints as “blessed.” It is not exactly a title, but a term of respect that recognizes their condition of beatitude in Heaven.

For example, the Roman Canon (Eucharistic Prayer I) says two paragraphs before the epiclesis (the invocation of the Holy Spirit), “In communion with those whose memory we venerate, especially the glorious ever-Virgin Mary, [...] blessed Joseph, her Spouse, your blessed Apostles and Martyrs [...].” It is similar with the Blessed Virgin Mary.

Similarly, on memorials and feast days of saints, whenever the saint is referred to in a prayer, he is addressed as “blessed” (beatus or beata). Here, for example, is the collect for today’s feast (July 3), the feast of St. Thomas the Apostole:


  Grant, almighty God,
  that we may glory in the Feast of the blessed Apostle Thomas,
  so that we may always be sustained by his intercession
  and, believing, may have life
  in the name of Jesus Christ your Son,
  whom Thomas acknowledged as the Lord.
  Who lives and reigns with you in the unity of the Holy Spirit,
  one God, for ever and ever.


That is probably the most direct reason that in English we have tended to use the term “Blessed Virgin Mary” rather than “Saint Mary.” As Raphael Rosch pointed out, in many other languages (especialy Romance languages), the etymologically equivalent word for “saint” simply means “holy,” and so it is more commont to hear expressions such as “Sainte Marie” or “Santa Maria.”