As far as I can figure, there's no technically good and reliable way to hide nsfw (or otherwise private) tags or files from a shared file domain. There are too many complicated ways that undesired words or thumbnails can pop up when you don't want them.
So, my current plan is to expand the 'local files' domain into multiple editable domains. Having completely separate partitions means all my tag autocomplete counting code and general searching code can guarantee stuff in the 'nsfw' partition won't pop up on a 'sfw' search.
This project will be on the next 'big things to work on next' poll, which will come after suggested tags and a faster dupe search algorithm are done. Please look out for it then!
For now, if this is very important for you, I recommend you run multiple separate clients, one for each distinct file domain you want to have.