I think by any measure you'll find criminals prefer USD, that's why over 90 percent of cash bills in circulation have drug residue on them. But honestly, the black market's willingness to use bitcoin in exchange simply highlights its utility as a medium of exchange.
Of course if it couldn’t be converted to dollars it would just be silly to assign it any value.
The practical effect of i’s use for criminal purposes is that it can be grossly devalued at any time by governments removing that utility.
Investors have to bear that in mind.
Fascinating development, I’d guess that blockchain has future.