But it's not "imperfect". It's 0. It's not one way in the slightest.
If I turn the lights off in my house and you can't see me through an open window will you tell me the open window is a one way window?
Imperfect implies say, 49% of light goes one way and 51% goes the other. Such a mirror does not exist.
And please remember the context (although I think the poster was embarrassed and deleted his original comment), we are talking about making black colors on metal, and he was wondering if it's because it's a one way mirror.
Hence the references to "imperfect" a few messages back. Pedantry of the kind you're exposing is dangerous (even life threatening) in real life.