Random Thoughts:
I heard this from a British Navy dive instructor, but don’t have direct experience with this technique. If your intent is to detect a tendency to panic, he claimed that removing the mask entirely and using a fabric blindfold was more effective than a blackout mask. He theorized that it had more to do with water contact with the nose than darkness, but who knows.
I know in freediving training they teach people to immerse their face without a mask and breathe through a snorkel for several minutes to trigger the “Mamalian Diving Reflex”. I have no idea if they are related but it is an interesting thought.
In my military training, I found that simulating black water was not nearly as bad as being in it up to my waist — but I had been in black water before so that may have skewed the perception.
Just curious, what do they teach regarding buddy divers in black water? We were on a hose with comms so it was never an issue. Also, what kind of information can you gather brail diving?