For the underworld, BTC is a very good currency. On the black market, it's already been for several years that illegal transactions with payment of BitCoin have been committed. Of course, petty criminals like quick profits, but for some, the anonymity of the transaction is very important. Now, in connection with the growth of currency criminals also want to get it. They generally do not care, the main thing is that anonymously and without intermediaries. If among those present there are such crimes, maybe they will open a small curtain and tell something)
Bitcoin leaves a digital trail. If you want to be truly anonymous, use black bytes by byteball. I do believe that if criminals are actually using bitcoin, the price will be wayyyy higher. This price is nothing compared to how much money is being transacted by mafias everyday. Sometimes real life is even more anonymous because you can meetup somewhere off the grid and you will not be traced by any CCTV. In movies, in order for them to have zero trail at all, they kill the people that knows about them or have a slight idea on what they are doing. In bitcoin, once you have sent money to an address, it will forever be in the public ledger. Anonymous or not?
I can't stress this enough but I do think that there's only a really small amount of criminals that are using bitcoin for their shady activities. First, if you have millions of dollars, are you going to put it in something that you are not really sure of the price? Are you going to put it to something which price goes up and down every single day. I think that people are exaggerated by the idea of deep web but that's just my opinion. I'm no deep web or bitcoin expert.