I was just looking around in the i2p darknet and stumbled upon this:
http://ylmulgfskl6uiwac4hw4ecwqdzd3oxtwaemzj25zc6k5q4rkexra.b32.i2p/en/trade/nbt_btc_360
It seems to be a working exchange and it has NBT trading pair. The exchange boasts to be the first darknet cryptocurrency exchange. Perhaps we should list them at nubits home page ?