Linguistically the response you cite translates the Latin Benedictus Deus in saecula. The benedictus in Latin, literally means to speak well or favorably about someone or something (bene = well + dictus = say or speak). Hence what we mean by “blessed” and the phrase “Blessed be God forever” is that “It is well that God should be forever praised.” We are not claiming to confer some sort of grace or favor upon God, as is often the meaning of the word “blessing” in English. 
 
Theologically though we can distinguish between God’s intrinsic glory and his external glory. As you point out, there is not one thing we can add or take from God’s intrinsic glory. God is glorious and blessed all by Himself and has no need of our praise. 
 
However, we can help to spread God’s external glory by our praise and acknowledgment of him before others, as well as by reflecting his glory through lives of holiness, generosity and conformity to the truth. 
 
In this sense we can also understand the phrase "Blessed be God forever" to mean, "May God's external glory and blessedness be extended and experienced in all places and times. May God be blessed (praised) everywhere, and unto the age of ages."

Organs and blood should not be sold and no Christian can seriously propose such a thing. In the first place, it violates Scripture which says, “You are not your own. You have been bought at a price! So, glorify God in your body.”. (1 Cor 6:19‐20). Hence, we are not owners of our bodies, merely stewards. We should not sell what does not belong to us. 
 
To be a steward means to use what belongs to another in a way that accords with the will of the true owner. Hence, we are permitted in charity to donate blood, and to donate certain organs while we live and other organs upon our death. These acts of charity conform to the will of the true owner of our body, God, who is love. Thus, Scripture encourages, “The gift that you have received, give as a gift.” (Matt 10:8) 
 
The second reason not to sell blood and organs is the harm that it does to the poor. If they can be sold, the number of those who simply donate them will decline. And the price of purchasing them will surely be high. This gives the poor less access to healing remedies. 
 
Hence, the buying and selling of organs and blood is an offense against Catholic teaching. It violates both the principle of stewardship, and also of charity.