You'd think that the privacy-focused coins out there are most used in the deep/dark web. XMR, Dash, ZCash, BEAM, etc. are the ones that come to my mind
I've never actually been on the dark web and don't plan to, but I've read a lot about cryptocurrency use there on bitcointalk. Silk Road was famous for accepting bitcoin and I think btc still remains the dominant currency for dark markets.
I agree that you would expect coins with more privacy features to be used, regardless of the legality of what's being purchased and I'm not sure why coins like monero and dash aren't very popular on the deep web. They can still be converted to cash but maybe not as easily as bitcoin can be...?
As in October this year U.S. and Korean authorities busted child porn criminal, they use Bitcoin as payment method.
It drive me crazy that bitcoin gets associated with that bullshit and that the association gets reported in the media as if bitcoin was the culprit, which it clearly isn't.