http://ysshlyesehcnscptskrpai4iupxccuvfcpbxk37kclbs3rsaqmolzaid.onion/darknets.html
(As a comparison with Tor: The onion service hosts acts as Tor clients, with a tunnel to the .onion rendevouz. By just watching what clients are always connected to the Tor network, the authorities should be able to create a list of clients that are likely running darknet sites.