Obviously there may be more than one cause of this. For me though, it was the temperature of my link box getting to high. I rearranged my vive area and when I did I moved the link box from sitting on a desk to being velcroed down to a windowsill. It wasn't the heat from the window, it was the insulating effect of the velcro on one side.
I noticed when I reset up the room I would get a blackout flash occasionally in random games, but it was rare so I didn't pursue it. When I got into ED recently I found it to be frequent when going into a visually busy station, and very common in CZ's, often in REZ as well. Once I removed that velcro that was covering one side the problem disappeared. The box is always far cooler to the touch now since I removed the velcro.
Since it's an easy thing to try, if you are getting blackout flashes look at cooling your link box down and see if the problem goes away
