[mike_d]

> Could a stack of greenbacks ever be "blacklisted"

Yes. Bill counters at banks capture serial numbers and associate them with your account when you make a cash deposit. Each night the serials are sent to a MCP database where they are checked against a hotlist entered by law enforcement across the country. The bank won't reject the bills on the spot, but depositing too many of the wrong ones will earn you a visit.

[ur-whale]

True, but you can still spend them at the grocery store.

Tracing cash bills is a complicated affair beyond a couple of transactional hops.

[AbortedLaunch]

The kidnapper/murderer Ferdi Elsas was found via bills spent at the grocery store.