Is it just my imagination, or is the mansion in Black Mirror and most of the property around it nearly IDENTICAL to the one in Phantasmagoria?

You're not alone with this feeling. I think that the makers of BM were heavily inspired by the Sierra game.

I was a maker of BM earlier this morning (too many beans).

Sorry, I couldn't resist. I tried to hold it in a long as I could.

I thought of that too... Plus, there is even a same puzzle in it. When you need to get inside the summer house inside the Walles castle and put a piece of paper under the door so the key on the other side drops and you can get it. This is the same puzzle when you need to access one of the locked doors in Phantasmagoria and have to put a newspaper underneath it to get the key on the other side.

But, even more so than Phantasmagoria, I think Black Mirror gives me Broken Sword vibes with all of these ancient tombs and traps you need to go thru in order to reach for the ancient keys. So, I would say they were also inspired by that franchise as well.