This would be a horrible idea.

First of all the FBI had put out a public notice allowing for anyone to claim ownership in the coins to do so and no one came forward.

Next, the concept of Bitcoin is that whoever controls the private key is the person who controls the subject coins. Since the FBI (I think it is actually the US Marshall service now, but this really does not matter) controls the private key to the subject BTC address, they are the ones that control the coins. To blacklist certain coins just because you think/feel they were "stolen" would create huge problems with bitcoin. Anyone could claim and present some kind of evidence that their coins were taken unjustly regardless of if the coins were stolen or not.

Third, once the coins are sold and disbursed to the buyers of the coins, the buyers will do their thing with the cons. They may keep some of them, they may use a mixing service to transfer some to another address, they may use some to buy goods/services, they may transfer some to an exchange to sell some. Assuming the transactions get confirmed then the majority (if not all) of the coins would be in the hands of someone who have nothing to do with the FBI nor had anything to do with the sale of the coins. It would be unjust to punish people just because they received coins from some unknown party (and would likely not have known prior to the transaction).