The darknet market failures have nothing to do with bitcoin. If anything, faith in TOR has been compromised, but it's still a very effective tool. DNMs are maturing and adapting, and still strong.
If anything, the DNMs use Bitcoin more in 2015 than in 2014 or 2013. In 2013-2014, there was a fair amount of interest in Litecoin, Dogecoin, and Darkcoin, with some markets using them exclusively. (Off the top of my head, Atlantis and Litebay for the former, and Doge Road for the middle.) Right now, every market I know of takes Bitcoin, with maybe a handful taking Litecoin/Feathercoin/Darkcoin on the side as a supported-but-hardly-used feature. I kept track of currency support in http://www.gwern.net/Black-market%20survival
And they all still use bitcoin.