People bought child porn with USD and GBP. It's only natural they will move onto a more "untracable"[0] currency. Also, just a thought, I was reading a link from HN about hiding messages and images in the blockchain[1]. Could this not be done with first tumbling some coins then hiding CP in the blockchain?
[0] - More so than USD.
[1] - http://www.righto.com/2014/02/ascii-bernanke-wikileaks-photo...