It's a good idea for an add-on service, which I think people are already working on: an independent database that keeps track of "dirty" coins.
Similar to the address labels on Blockchain.info, I guess. I like this idea better because it lets people choose to participate.
There are many reasons why that would not be feasible. For example, deciding who gets added to the blacklist would require some kind of presentation of evidence and then perhaps a vote.