I don't think a lack of fiat cash will stop the black economy....

I don't think a lack of fiat cash will stop the black economy. If reports are correct there are too many people that want an anonymous exchange vehicle (drug users/sellers for one) for it to disappear. If necessary, people will invent their own currency that governments cannot (easily) track. I'd say even governments authorities use cash at times to pay informants etc. If necessary people can use barter of goods, provide food/accommodation etc. It may make some things more difficult for a while but that is all. When pressed, ingenuity comes to the fore.