Why not try it? It should be easy to implement. Instead of arguing, let's just try it. You think it won't work, great. Let's try it.
Yes, go ahead and implement it. Here's a thumbnail sketch of one way to start:
+ Create a web service that lets anybody upload their email address and a list of public keys.
+ Send the user an email whenever 'tainted' coins are sent to any of those public keys, telling them how tainted they are and where they came from.
That's it.
For extra credit, you could let users upload their wallet.dat files (private keys encrypted, I would hope) and auto-extract all the public keys in the wallet. Heck, if you stored the private-key-encrypted wallet.dats you might be able to charge a little for both blacklist detection and wallet backup.