Blasphemy is speaking against God or the Holy Spirit, such as the pharisees attributed Jesus' casting out demons to Satan. Using the title "God" in a meaningless byword isn't near as bad, and should not be considered blasphemy. But "taking the Lord's name in vain" is not blasphemy either. If a person says "thus says the Lord" when the Lord did not speak, that person is taking the Lord's name in vain, because they are claiming to speak on His authority when they don't have it. The word "name" in this context means "by the authority of," much the same as baptizing "in the name" of God. So when someone teaches false doctrine in the guise of "it's God's word," they are taking God's name in vain. When a person quotes a verse of the Bible out of context, in which the meaning is twisted into some idea not conveyed in the text, that person is taking God's name in vain, because they are claiming to speak the truth from God's word that is actually not true. Saying "Oh my God!" as an emotional expression is widely considered "taking God's name in vain," but it isn't, in my opinion. The expression implies calling on God to right a wrong, or to fix a problem. Someone could use that expression vainly or meaninglessly, or they could use it with the thought of praying that God act on the situation. Therefore, the expression itself is not sinful.
TD