The reporting I've seen was that someone in the crowd was killed by this, just not Trump himself, that implies that whatever was fired at least was deadly.
Not that I agree with the arguments here either way but... Devil's advocate.
Blanks are not a "safe" round and are deadly. They still fire a regular powder charge and an explosion still leaves the barrel, they just aren't pushing a bullet ahead of them. They will kill just like a regular round at closer ranges. An attendee dying doesn't inherently mean that blanks weren't used. Actual bullets are evidence, but we don't have concrete info of that sort yet.