And the black market is a key safety valve for a fully-functioning capitalist economy (like it or not).

Hell, even the USSR had paper cash in circulation allowing some privacy and trade between individuals.

Having solely account-based money under complete regulatory control amounts to a huge increase in what is practical to put under political control. It could drastically increase the incentives to ban and regulate trade in anything of interest to special interests. E.g. the pharmaceutical industry could finally get rid of herbal medicine.