Would it bring more attention to the matter, or would it be closer to blackmail?
Both, obviously. If you are trying to force the person to pay closer attention by making sure his boss is aware that the question is asked, that is a form of blackmail.
Does that necessarily make it a bad move? Not always, but give a lot of thought to the personality of the people involved before you take this step. Is this the kind of person who regularly ignores emails? Are they likely to think poorly of you for taking an aggressive approach? Is their boss the kind of person who is offended by breaches in the chain of command?
It also depends a lot on the structure that you're both within. In particular, I am more inclined to CC my boss than someone else's. Because he needs to know if I'm being slowed down by someone else's lack of response. If my boss happens to be the boss of the person I'm chasing, I'm not going to let that stop me.
But, more often than not, there is a better way. Follow up with a phone call, or go to the person's desk, before you start playing the political game.
As with all office politics, the question for me is not "is this appropriate?" The real question should be "will this achieve the result I am looking for, without costing me a working relationship?" And that always depends on the personalities involved. There is no hard and fast rule.