Illegal dark web markets use BTC as currency. The BTC entering and exiting these markets is highly tumbled, making it very hard to track.
FYI dark markets often use XMR (monero) now, due to it's true anonymity. That's why a few governments around the world are one-by-one banning it's sale (see Kraken and the UK government monero situation in september 2021). Bitcoin was figured out by most big governments quite a few years ago, and dark markets are quite quickly moving on to XMR.