This is an old thread now, but I thought I'd reply anyway. Someone might find it useful.
About thirty years ago, I was in the Australian Army Reserve, this was back when they still used the 7.62mm SLR. The blank rounds for them looked almost like they were made out of black plastic. They ejected out of the weapon looking similar to the way they'd gone in, except with the crimped part blown open.
They tended to leave a lot more residue in the barrel and reloading mechanism than a real round, so if you have the weapon that's something to consider.
Also, as Bob said, it's possible to kill with a blank round. During my recruit training they demonstrated the force of one by putting an empty Coke can over the muzzle and pulling the trigger. It obliterated the can.
Steve