I work at a hospital, and the number is blocked.
I guess your justification makes a slight amount of sense (not knowing someone is undergoing medical treatment)
But honestly, why would anyone assume that's the reason at first?
Personally, my first assumption would be "someone I[or the person whose phone it is] know is in the hospital and needs to get a hold of me[or person whose phone I'm looking at]"
Working at the hospital and having applied to other hospitals, it's a pain to just see "unknown" when they could easily put "Hospital for Sick Children" "Sunnybrook" "Lakeridge Health" "Rouge Valley Health", etc. With the main switchboard # attached.
That gives you zero patient information, aside from the fact that the hospital has your # on file, which could be for any number of reasons (the most common being the one listed above) and allows you to identify if a call is important or not.