Using a mixer might be a good solution to avoid be connected into the dark marketplaces, but why to use it bitcoin already does proctect those people under their adress wallet.
You must use a bitcoin mixing service if purchasing goods off the darkweb because bitcoin is easily traceable via the public blockchain, any coins you spend can be traced all the way back to where you purchased those coins from, if you used a service such as Coinbase to purchase your coins then you would had to have given them your personal detail at some stage and that's how they will get you.