In the Old Testament, blessing is a communication of life from God giving vigour, strength and success. The effect of the blessing, most often, is fertility: "Be fruitful and multiply" is the blessing received by Adam and Eve, as well as by Noah and his sons while Abraham's, Isaac's and Jacob's blessings include innumerable descendants. God not only blessed people but also the seventh day, making it fruitful, a source of blessing.
Only God can bless; humans bless only by praying that God will bless someone or something. That blessing, in the Old Testament, is invoked by one who, in some way, represents God. It cannot be taken back or cancelled even if obtained by fraudulent means.
The story of Jacob stealing his brother Esau's blessing (Genesis 27:28-9) is evidence of this because, the father's life, strength and authority were passed to the son through the blessing. The laying on of hands often conferred this blessing and so the blessing flowed from one person to the other.
As God blessed more than just humans, so men did too. David blessed his house after the enthronement of the ark (2 Samuel 6:20). Solomon blessed the people after the Temple was dedicated (1 Kings 8:14, 55). The priestly blessing of Numbers 6:22ff is used in the Jewish liturgy of today.
In the New Testament, we read of Jesus blessing the loaves and fish, the bread and wine at the Last Supper, the Apostles at the ascension, etc. Mary is called "blessed" among women and her hymn is one of blessing and praise for what God has done for her and for all of humanity. "Blessed be God" is frequently used in the Epistles. It is recognition of a benefit conferred by God and so it is a form of thanksgiving.
We, too, bless one another.
The priestly blessing has been considered special as has the blessing of parents upon their children. We bless the palms on Palm Sunday; we bless the ashes on Ash Wednesday.
In addition to that, Catholics have their homes, their cars, religious objects such as rosaries, etc. blessed. In these cases, blessing becomes more of a prayer for God's special protection and presence in our lives.
God blesses humans but humans do not bless God, although the phrase "bless the Lord" is frequently used in the Old Testament, especially in the Psalms and books such as Daniel. After several verses declaring that God is blessed, the hymn sung by the three young men who were cast into a fiery furnace, calls upon all nature and humanity to bless the Lord: "Bless the Lord, all you works of the Lord" (Daniel 3:57ff). As the second part of each verse testifies "praise and exalt God above all forever," it is really an acknowledgment of God's power and glory and is used to express gratitude and praise to God.
So when we say, "I adore you, I love you, I bless you," we are not thinking of our bestowing blessings on God. Instead, we are praising and thanking God who bestows blessing on us. We could replace the words "I bless you" with "I praise you, I thank you," adding them to "I adore you, I love you."
In the Eucharistic Prayer at a children's Mass, we were told to respond at intervals: "I adore, I love you, I bless you." How can we bless God?