It is a dogma of faith that the saints intercede for us, and certainly the Blessed Virgin Mary is included certainly, per the Immaculate Conception and Assumption dogmas. Also from Catholic Encyclopedia: "To intercede is to go or come between two parties, to plead before one of them on behalf of the other. " Yet, some want to restrict the use of mediation only to Christ and the word intercede to angels and saints.
Council of Trent (Session XXV):
the saints who reign together with Christ offer up their own prayers to God for men. It is good and useful suppliantly to invoke them, and to have recourse to their prayers, aid, and help for obtaining benefits from God, through His Son Jesus Christ our Lord, Who alone is our Redeemer and Saviour. Those persons think impiously who deny that the Saints, who enjoy eternal happiness in heaven, are to be invoked; or who assert either that they do not pray for men, or that the invocation of them to pray for each of us is idolatry, or that it is repugnant to the word of God, and is opposed to the honour of the one Mediator of God and men, Jesus Christ