I think the transparent money idea is a clever one. Bitcoin does this although I'd say it's a side-effect rather than a feature and you have to accept a whole lot of baggage. Applying this to money would be clever. I haven't thought it all the way through but it feels like you'd get tripped up by the fungibility of money. If I use some of my tainted dollars to get a loan and then use those loaned dollars to buy something from you, haven't I bypassed your filter? (As long as they guy making the loan doesn't mind getting paid with tainted dollars.) You could try to make the loan provider tag their loaned dollars with the tag of the payment, but I'm not sure how you'd enforce it. Not to mention that fractional reserve banking means that "new" dollars are being created and destroyed all the time. Still it's an interesting idea and bears further thought.