There is already too much infrastructure made for bitcoin in darknet. You have blockchain exporer there, escorove services, bitcoin exchanges, mixers, scammers, etc.
None of the ppl involved in these services, e.g., mixers, will allow to have any other crypto to take their mixin cut.
Until a company like SABR.io posts a detailed mega-file of hundreds of darknet user's recent activities as proof of concept. Using a public ledger for a DNM is about one of the most absurd things ever when a vastly superior alternative exists.