Anyone who has had even a little training knows that survival in a self-defense situation where lethal force is used is almost always based on fast, accurate shot placement. Once you pull a gun, empty or loaded with blanks, lethal force is in play. That is not to say that just presenting a gun won't stop the fight. Just don't bet your life on it.
I would not bet my life on it either, but the reality of guns and lethal force is that survival in a self defense situation is most often
NOT based on fast, accurate shot placement. The estimates of 500,000 to 3,000,000 times a year guns are used for self defense each year
http://www.cnsnews.com/news/article/cdc-study-use-firearms-self-defense-important-crime-deterrent doesn't jive with fast, accurate shot placement. It jives with just having a gun. Beyond that, of the self defense situations where triggers are pulled, there are lots of times when the bad guys are not hit. Tremendous numbers of shots are fired that never hit the intended target, but still manage to stop the threat.
By and large, most people are not great shots under stress. Heck, based on what I see at the gun range and LTC classes, a lot of folks are poor shooters when not under stress. It doesn't help that many police departments think they are doing good if 30% of their rounds hit the bad guys.
No, you can't count on having a gun or inaccurate fire to stop a threat. That is true. However, fast, accurate fire is not a dominant parameter of self defense shootings in general.
Still, the blank is a stupid idea that comes with all the legal ramifications but none of the lethal benefits. If the guy thinks he needs it as a safety device to keep from having a ND, maybe he should not be carrying a gun.