Not true.  This change to the client that blacklists those coins does not follow the Bitcoin protocol.  It rejects blocks that are valid using the client prior to the change, therefore this client is imposing a change to the protocol that is not backward-compatible.  (It is still forward-compatible though, meaning clients that don't implement the blacklist will still validate and accept blocks solved by this "blacklisting client").

Miners will be taking a risk by adopting a client that uses the blacklist.  They risk that there truly won't be enough miners using the blacklist.   If this happens, there will be a blockchain fork and those miners using the blacklist will be working on blocks that may not ever become part of the longest chain.   Miners are not gamblers -- they will not switch to a client that is not backward-compatible without first being really confident there is almost no chance the changes will cause a blockchain fork.

Even if there were something like 80% of the miners onboard, however, that doesn't guarantee success.

See, the buyer of these disputed coins has an economic incentive for them to be fungible.   So all this party would need to do is to spend some of the coins and include significant fees on those transactions such that miners are leaving a significant amount of money on the table by implementing the blacklist.   So if the block reward paid to miners is 25 XBT, then maybe adding a transaction fee of just a few bitcoin would be enough to sway over some miners such that the longest chain continues to extend off a block which includes the blacklisted coins.  That action requires no change to the protocol and no change to the client.  And once these transactions start to circulate, a blacklist on taint starts to harm others who are innocent and thus there's more "backing" available to ensure that the blacklist fails.