Bitcoin and the dark web have little transparency and piss-poor security. They are crime waiting to happen. Maybe that was the point all along. However, perhaps they forgot about the big sharks, as this 4 month old story about the theft from Sheep Marketplace might show.
Wall Street dark pools are private shadowy stock exchanges. They sometimes get raided by hostile HFT programs and the like. Those with the competency to raid a dark pool would no doubt find bitcoin and dark web sites trivial to rob.
Dark net drug dealer Sheep Marketplace shut down in December 2013 after $5.4 million in bitcoins were stolen. This highlights the huge and overwhelming problem with bitcoin and the dark marketplace. There is no way to determine if a theft actually happened and how it actually happened. The bitcoins could have been stolen, or maybe they stole the money themselves or allowed it to be stolen for a cut. There is no easy way to know, no audit trail, and no regulators who can move in quickly to look at the books.