Unlike other social platforms (Facebook, Twitter, Instagram, Pinterest), Tumblr has no structured notification to let you know when you’ve been mentioned on other Tumblrs.
So we resort to add-hoc “mentioning” systems with Tumblr tags. (Why this doesn’t work/how Tumblr tags/search is broken is for another time.)
It looks like Tumblr is making moves to fix the lack of mentioning system, though.
If you start typing in a Tumblr username with an @-, Tumblr will search, autocomplete, and automatically make a link to the user’s Tumblr.
It will also remove the “@.” (The @ triggers it, but does not remain.)
Unfortunately, this is not complete “user tagging.” From what I can see, the user receives no notification their Acitivity feed that you tagged/mentioned them.
But one imagines staff is working on adding this. (See what I did there?)