Doesnt silk road have a forum ?
There are hundreds, probably thousands if you count the foreign language ones like the Chinese 'dissidents' of forums that have a .onion address.
Publishing through TOR might not be enough to prevent hardcore investigation succeeding but it might be enough to stop you getting put on "a list". Problem is, using TOR probably gets you put on "a list" anyway.
Some kind of anonymous gateway might be fairly cool but the problem is anonymity is a double edged sword. Just ask (name escapes me), an old anonymous email service that got shutdown because of abuse (I want to say anonnet.fi but that's incorrect)
Tormail, they are back now the Russian host they use to use closed down not Tormail itself.