Bitcoin is already legitimate. Just because people use it for so called illegitimate means doesn't mean anything. People use fiat money for this type of stuff and in far bigger volume. If anything, its use on the darknet proves its legitimacy as a currency because as with any currency you should be able to buy absolutely anything you like with it.
They like to put Bitcoin in a bad day light, you must know by now how the media is. Just ignore these clowns. Bitcoin isn't any worse than fiat currency regarding being used for criminal activities.
Fiat would have been fake when Charles Ponzi created a big ponzi and many people smuggled. 
