Someone on another forum had scanned a raw negative so just the grain showed up. In other words, it was a blank frame.
I have the JPG...somewhere
It was Fuji 400 B&W something. Neopan maybe? Very grainy, although fairly fine.
But anyway, use a layer mask in PS and adjust the opacity to suit. Since the negative scan has some bubbles from development, you may want to either rotate/mirror it or stamp them out entirely or every picture will have the same elements in the same place.
I can try and dig it up if you'd like, or perhaps someone on here has done something similar with other films. This is the most convincing way to do it, for me, though I want to see what other options are out there
Since you have Efex, isn't there a plugin called "Silver Efex" or something? Or is that just for B&W?