I don't see how coins could ever be irrevocably blacklisted unless Bitcoin itself was hijacked in which case it doesn't matter any more because it would be worthless.
There's no shortage of voluntary blacklisting going on right now. That's easily sidestepped with a few nifty moves.
The amount of Bitcoin users is still low enough and committed enough to make a real difference by voting with their feet and shunning those who blacklist. In most cases though it's not the blacklisters who want to be doing this, it's The Man breathing down their neck.
But there might be a way to check coins against a list of scammed coins. What would you do when you receive coins? Checking them? Or don't care and risking that you have problems with these coins?
In fact this can be done very easily and it only needs some who fear getting such coins to become a problem.
