Well you gotta take the good with the bad with the DarkMarket. Sure, maybe a Dallas Buyer's Club scenario might seem more legitimate to the average person (or a market for any legitimate pharmaceutical drugs at lower prices because a huge markup in Europe / North America). But what about all the other nasty, illegal stuff will this be used for? The creators of this can't control the goods and services that are sold any more than the creators of bittorrent can control the files being transferred around.
Sure, maybe some drugs seem harmless and you can argue they're a victimless crime or whatever. What about all the other insidious shit out there that this will be used to peddle? CP, hit-man & assassination services, weapons, slavery, identity theft and stolen credit cards... surely the concept of DarkMarket opens up a new market for all these types of transactions too - you can't expect these services to be somehow excluded. In my utilitarianism-based opinion; this is a far greater harm than good for the world; the benefits of being able to buy some recreational drugs or make purchases anonymously is heavily outweighed by giving the rich the ability to put out a hit some some journalist with a dissenting opinion (with a much lower chance of being caught than without this service), or a new market for CP, etc..
I wish the creators of this project would realize this. Maybe this is just somehow acceptable in the worldview of the anarchist or the libertarian-leaning types. Or maybe it's morally rationalized by some thoughts on the lines of "Hey, I didn't kill those thousands of people! I only sold those guns to that corrupt African warlord" or "I only helped create the software that assist people making illegal transactions, I didn't actually sell or make that CP" and somehow absolve themselves from guilt because they're not the ones making the transaction. I might not agree with the current laws on drugs, but I certainly don't support this project.