Usually for me, the way I dissociate in session, if something triggers me, is to just space out, go 'somewhere else'. I know I'm feeling stressed or panic, but it numbs it for me. Usually when someone asks me where I am, or what's going on with me, etc., I can't come up with an answer. If it was me feeling like that, it would be dissociation. Some people just consider dissociation switching alters, or whatever, but it can also be spacing out.

I haven't used a light bar, is that part of emdr? Usually when we do that in my session, I just follow my t's hand as he moves it back and forth.

Did you have any thoughts, images or sensations (touch, smell, sound, etc) that happened at the same time? It could have been that some memory was triggered, and your brain is trying to figure out what's going on. Another thought is that I was doing some vision tests and was told that because I was watching the light without blinking, I started to black out my vision. Is it possible the loss of vision was because you weren't blinking, and then it triggered an emotional reaction to lose your vision? Just some thoughts to throw out to think about. Perhaps they will help you figure out what happened.
be well,
IJ