Blessings are something we use a lot in the church in many different ways, making it hard to sometimes know what blessings actually are and entail. To make matters vaguer, The Oxford Dictionary of the Christian Church defines a blessing as "the authoritative pronouncement of God's favour."
One of the ways we see this pronouncement of God's favour* is through the setting aside of people and objects for a specific purpose. In the Rite I Eucharistic Prayer, the celebrant asks God to "bless and sanctify, with thy Word and Holy Spirit, these thy gifts and creatures of bread and wine; that we, receiving them according to thy Son our Savior Jesus Christ's holy institution, in remembrance of his death and passion, may be partakers of his most blessed Body and Blood." In other words, the blessing sets aside the bread and wine to be for us the Body and Blood of Jesus Christ, even if there are differing opinions on how it does so.
We also bless people in The Book of Common Prayer to set them aside for God's purpose. For some, it is a particular calling, such as the blessing present in Marriage, both of the couple and their rings. There is also the blessing of the worshipers at the end of Holy Eucharist, signifying the specific purpose God is calling us to as followers of Christ Jesus out in the world.
These are just a few of the ways we can use blessings to set aside people and objects for a specific goal to serve God in the world. There are many other objects we can bless, as well as people in particular roles we can bestow blessings on, in the church. In this way we can set them aside for whatever God intends to use them for.
*or "favor" depending on what side of the world you're on