A dollar bill is not as fungible as you think. All bills are numbered. There are blacklists of bills (e.g stolen from a bank heist). When you are detected giving such a blacklisted bill to the bank, you will be asked a few questions, but the bill won't loose its value. Same could happen for blacklisted Bitcoins: as Mike and others have described, they could get white-listed again if you are willing to identify yourself and can make plausible that you are not connected to the reason for their blacklisting (e.g. by showing a receipt that says where you got them from).