Dark marketplaces are mostly considered bad but I think that they can be good if you think about it, its not that they are doing a bad job in the bitcoin economy as a example and it made the bitcoin also more popular than ever right ?
Yes the darknet markets have had a big impact on bitcoin but so has wikileaks, without those two things in the early days who knows what might of happened to bitcoin, they sure helped launch things.
I agree most likely it never would came so far as it has been now, it had some bright sides but is now pretty negative for the bitcoin.