I'm not sure that makes any difference. I'm not sure how big this market is, but it could plausibly be big enough. It would probably leak into other markets too. Drug dealers comfortable with bitcoin might also buy shoes with bitcoin etc.
And I'd be able to buy bitcoin from drug dealers trying to convert bitcoin into cash, buy shoes from someone with the bitcoin, and that someone could sell that bitcoin to someone looking to buy drugs from a drug dealer. The demand for bitcoin naturally smears over the entire market for bitcoin.
