This was a funny article in Slate that punctuated the problems the dollar is having — Euro Trash:
Generally, the largest denomination of U.S. currency readily available is the $100 bill. But in the past two years, the European Central Bank has started to print 200-euro and 500-euro bills. These larger bills thus allow for the concentration of wealth in smaller packages. At today’s rates, a 500-euro note is worth $682.
So if you wanted to, say, hide cash by swallowing it temporarily, euros would the obvious (and more comfortable) way to go. And indeed, as Grant notes, in October a drug mule traveling from Spain to Colombia was found to have an unexpected form of contraband in his stomach: $197,000 in euro notes.
Basically, the black market has a big influence the world currency of choice because they distribute it throughout the globe in high numbers so efficiently. On the positive side, they also don’t spend this money as freely, because they tend to hoard it, so the effects may be neglible in the short term. However, the overall implications of a major black market shift from dollars to euros is unsettling in itself.