Yeah, some secret shops on dark marketplaces having bitcoin as the payment for the transaction, because it's anonymous and easy for online transaction.
Indeed thy can mix it up so it gets untraceable, its easy for them and its also that they need to be anonymous for other people.
I think its a good option and that for some things its not even bad on such marketplaces.
Question is, who needs that? I think for the masses the part which makes your transaction untraceable is not handy for the masses. We want bitcoins to pay for things. That's it. Having the transaction being ultra dark/anonymous is a feature, but by far not the most important feature.
That is the feature of bitcoin that it is anonymous and you can't even try to trace each transactions. And that is why people in the dark market wants to use bitcoin as medium of payment for their transactions because they want it for their safety. They don't want people to know their identity and wanted to keep their identity hidden always.
Bitcoin transactions are not annonumous.