I took a look at dark markets, fascinating stuff of course, I strongly oppose drug usage, but you can't stop the progress.
What I've noticed though that no anon coins are used there. It's just Bitcoin (and sometimes Litecoin). No Dark XC Cloak Monero and so on.
It proves my belief that all anon coins are more or less marketing gimmicks with no real world usage, and very unclear perspectives.
Also they are fragmented in many different projects, which undermines their whole ecosystem, because obviously the only chance to succeed for them would be some unification of efforts.
Don't invest in anon coins

This is my opinion I might be missing something. Please share yours.