How does regulation work for illegal drugs, prostitution, or other black markets, which can overall amount to double-digit percentages of economies?
Bitcoin only needs the black market to be a huge success. If it can avoid any surveillance ramps, that is ideal. It doesn't need to be everywhere.
IMO, the surveillance on/off ramps are going to have a hard problem ahead of them, of their own making. Black markets are only going to accept "cleaned" bitcoins because they won't put themselves at risk of being surveilled. The "cleaned" bitcoins will not be usable on AML exchanges. The "unclean" bitcoin can be cleaned through CoinJoins and such, at the risk of whoever obtained them in their unclean state, but once cleaned, they'll be back on the black markets.
Trading fiat currencies for Bitcoin will happen mainly in person, with cash, to avoid the surveillance.
You will be able to tell, to an extent, whether bitcoin are clean or unclean through their transaction history - a clean coin will be one which originates as the output of a CoinJoin. All transactions on the black markets will be a CoinJoin of some kind.
For most purposes, this won't matter because the majority of trade will be conducted over the Lightning Network, which has sender anonymity, and will very likely get receiver anonymity in the near future. Node operators can use Tor to avoid being surveilled - and the transport protocol used is fully end-to-end encrypted and authenticated.
Since the majority of trade will be happening on the black markets, exchanges which built their castles in KYC land will eventually fail. Nobody will care for "unclean" bitcoin if they are too heavily regulated. There will probably be a premium on clean bitcoin due to the risk involved in anybody who decides to wash unclean bitcoin.
Savers will not care if they have to resort to black markets to protect their wealth from theft or seizure. As long as they can find some local black market vendors who'll trade them for some other goods or fiat cash.
For governments to attempt to stamp out the black markets, the solution involved will be much worse than anything you could imagine being caused by Bitcoin - complete totalitarianism is the only way it could be enforced, and your open internet will be finished long before then. Careful what you wish for.