Nobody you’re dealing with on the blackmarket is going to want to wait for the transaction. In some way shape or form you’ll have to access your online coin wallet during the transaction, and if you’ve done any “legit” stuff with that wallet the government could know to be monitoring it.
Of course that’s just the government side. Remember if you’re doing black market stuff then you’re dealing with criminals, one rule of back alley buying is never bring more money than you’re already going to give the guy, because if they see it they’re probably going to want it, and they are after all criminals. Now what happens if the guy knows you have bitcoin info on you, online or off, there’s probably more money on the other side of that than you’re going to give him, it’s like telling him to follow you to the ATM.
If cash ceases to exist the black market will become barter, or will invent their own cash that’s still physical. They won’t be using anything that’s basically a debit card because they will (wisely) not trust each other.
That remains to be seen. The way it goes in the black market is that it adapts to the society that it’s in. And if it’s in a highly technological society, then it will operate in the same venue.
How that is “worked out” in detail is never seen “beforehand” - but it does get done in the end.