Non-Christian here. I have heard that "God damn it" or "God damn [that person]" is blasphemy because you are telling God what to do. But other interpretations are that it's only blasphemy to misrepresent what God told you to do, like "God told me to hurt that person".
Is “God damn it” blasphemy?
The short answer is yes.
The fourth commandment prohibits taking God's name in vain, that is to say using his name in a light or frivolous manner rather than giving proper respect to Whom He is.
It is definitely below the dignity of a Christian to wish someone damned for eternity. God desires the conversion of sinners and not their condemnation to Hell forever.
Thou shalt not take the name of the Lord thy God in vain. His name, according to the commandment, is both “Lord” and “God”. Those who address him in prayer (see the Psalms as reference) call him “Lord” and “God”!
In general, the Second Commandment governs this area: "Thou shalt not take the name of the Lord thy God in vain" (Ex 20:7; Dt 5:11). Specifically, a person must have respect for God's name. Throughout Sacred Scripture, God's name is held sacred. He reveals His name to those who believe, and through this revelation, invites them to an intimate and personal relationship.
For example, in the story of the call of Moses, he asked God, "...If they ask me, 'What is His name?' what am I to tell them? God replied, 'I am who am.' Then He added, 'This is what you shall tell the Israelites, 'I AM sent me to you.' God spoke further to Moses, 'Thus shall you say to the Israelites: 'The Lord, the God of your fathers, the God of Abraham, the God of Isaac, the God of Jacob, has sent me to you. This is my name forever; this is my title for all generations'" (Cf Ex 3).
Therefore, we rightfully use God's name in prayer or in other ways to bless, praise, and glorify Him. The respect for God's name reflects the respect a person owes to God Himself (See Catechism, No. 4142ff.).
Given this foundation, certain forms of abusive language are sinful. First, to abuse God's name, whether the word God, Jesus, or in some other form, is objectively mortally sinful. The same rule applies to abusing the name of the Blessed Mother or the saints. One has to ask oneself, "Why would someone use the name Jesus as an expletive when angry or impatient? Would not such an action show an arrogant and disrespectful attitude toward God, whom we should love above all things?" I often wonder what a Moslem must think when he hears a Christian use God's name in such an irreverent and improper way.
Second, blasphemy is also a sin. Blasphemy is contempt for God, expressed in thought, word or action. To use words either vocally or mentally against God which show hatred, reproach, disrespect or defiance is sinful. This prohibition also applies to the Blessed Mother and the saints as well as sacred things or Church related practices. Moreover, blasphemy includes invoking God's name to legitimize crimes or harmful actions against others. The Catechism notes that blasphemy is a grave sin.
Third, cursing is to call down evil from God, and usually involves specifically invoking God's name, not just His power. For instance, all of us have heard someone say, "God damn it," or even "God damn you". Here a person is commanding God, who is all powerful, all good, and all just, to damn someone (or something) in Hell for all eternity. Who are we to ask God to damn anyone or to bestow some evil upon them? Objectively, this act is a mortal sin.
Finally, profanity itself is wrong, even though such words may not specifically involve the name of God. God gave mankind the gift of language which should be used positively. Language should build good relationships with other individuals, and enable people to share their lives intimately with each other. Sadly, more and more, we hear in normal conversation profanity especially those four letter words, like s*** and f***. We also hear people speak profanely about good and holy topics; for example, they profane human sexuality or the act of marital love. Such language is not only negative, vulgar, impolite and offensive, but also debases the dignity of each human being. Moreover, this language reveals not only a person's bad attitude and lack of respect for others, but also his own immaturity and insecurity in dealing with others. In using these words, the person builds barriers rather than bridges with another person.
Each of us needs to be careful in our use of language. Objectively, using God's name in vain, cursing and blasphemy are mortally sinful, although lack of due reflection or habit can diminish culpability. Profanity too can be mortally sinful when done with great anger or viciousness against another person. Each of us needs to control our tongues, and prevent a bad language habit from becoming part of our person. St. James wisely admonished, "Every form of life, four-footed or winged, crawling or swimming, can be tamed, and has been tamed, by mankind; the tongue no man can tame. It is a restless evil, full of deadly poison. We use it to say, 'Praised be the Lord and Father'; then we use it to curse men, though they are made in the likeness of God. Blessing and curse come out of the same mouth. This ought not to be, my brothers!" (Jas 3:7-10). - Why is bad language a sin?
Be careful what you say, it may come true!