In the real (non virtual) world receiving stolen goods is a crime whether you know the goods are stolen or not, (the goods in question can be banknotes) - if bitcoin can be classified as goods and if the coins can be traced back through the blockchain to the point where they were stolen then they could quite legally be subject to seizure and returned to their original owner in the UK. To avoid this happening to unsuspecting individuals they would need to be temporarily marked until such time until they were received by the original owner when such mark could be removed.
That is quite interesting, specifically when in comparison to the US. Here, possession/receiving stolen goods is a burden of proof placed on the state before any charges can be brought against the receiver who unknowingly received the stolen goods.
Of course, the unaware citizen must truly be unaware (able to defense his position if called upon in court), but the mere possession of stolen property (given it truly is) is not technically illegal if the one who possesses it did not know.
Can a US court take stolen property off a citizen who paid for it unaware it was stolen?