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.