As I'm sure you've experienced in the past, some people are so stupid they even lie to their defense attorney.
I'm not sure how "I was only shooting blanks" could be part of a defense, but if it is, I would tread carefully. If it is a semi-automatic firearm, it would have to be modified to properly cycle with blank ammunition. I would expect one of the questions to be "Oh yeah? Where do you get your blank ammo?"
I've only seen blanks offered for sale at local gun stores in rimfire calibers for use with training dogs and such. Blanks are widely available for common guns such as 9mm pistols and etc, but I've never seen them for sale at local gun stores or shooting ranges. The most common source of supply would be places that produce blanks for the entertainment industry.
Could we possibly have a translation problem? Many folks shoot Airsoft guns, that are quite exact replicas of real firearms but aren't firearms.
You may be limited in what you can share, but we may need some more details to provide cogent information.