Rather than blacklisting coins or anything along those lines I would prefer to see the major mixers (blockchain.info, inputs.co, etc) at some point develop a system to work with law enforcement and courts to prevent the mixing of coins that have been used in serious or violent crimes. Anonymity is a crucial aspect of Bitcoin but at some point there will have to be more co-operation between law enforcement and corporate Bitcoin services and exchanges.

Whatever the tor mixer is called (bitcoinfog?) that is used almost exclusively for the silk road will never comply, and competition will arrive that does not use any system like this. Some mixers could be created by criminals themselves fairly easily. However I believe that providing some system of regress or  tracking for coins used in crimes such as those theorized in this thread is likely, if not inevitable among Bitcoin businesses that wish to operated legally in most countries.