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?
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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.