They won't hit the market one day , they are circulating since the time Bitcoins was created. The people generally take a hold of Virgin Bitcoins to make them non-trackable , BUT , considering how less of these coins are in the market most of the coins held by the dark market and scams are coins which one of us might have possessed at any moment.

But at least unlike cash , We can track them ! Contrary to popular belief , I do think Bitcoins and other cryptocurrencies even though have so many coins in the dark market and scams are way better than cash , we can at least track them , check their history , most wallet companies do have KYC, this is what comes with regulation.

Stolen Bitcoins are something that can definitely be tracked if given the right person to do the job , most of the times problem is , people don't hire the right ones , I do think the society will be benefited much if they started hiring hackers , people who are notorious for doing these things , this way you can provide them with :
-Reduced sentence
What you will get:
-An experienced person who did only this.

I do think one has to intelligently use the resources that they have because Bitcoins have wide scope .