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I've noticed that all the various NerveGear iterations in Sword Art Online have three LED's on the left side, labeled POW, WAN, and BLK.
I think it's safe to say that POW is the power, and WAN is the internet connection. What is BLK?
There is nothing confirmed as of now, however we can see that the most applicable name to give according to online discussions is Brain LinK (BLK)
The blinking may suggest a weak link or solid colored a firm connection and so on.
You see it in season 2, episode 12, 16 minutes in. When Kirito is fighting death gun, Asuna enters his room and the BLK light is blinking blue. Therefore, BLK is the Brain LinK in it's activated mode.