Oh, I know it's wrong, St. Jude, I just don't know if blasphemy is the right word for it. It's a sin against the 3rd Commandment, that's all I know.
As I was tapering off and learning not to use the Lord's name in vain, I'd sometimes say things like "G-- forbid," making sure that I really meant it, that I was really hoping that God would forbid what I was speaking against. But that felt too much like an oath as well.
"I swear to G--" would be doubly sinful, I think, because Catholics are not only not supposed to take the Lord's name in vain, but we're not supposed to swear.