I need to see the sopace where you are doing this and understand the reason since a big mirror is just as awkward to design around than a big TV.

Well, I can see a benefit: when the TV is off, you just see the mirror, which in my opinion, is more attractive than the TV. But I can't answer the OP's question.

Friends did this, and it didn’t make the tv too dark. The mirror was framed beautifully and I had no idea it was anything but a mirror until she turned the tv on. And (gasp) it was over the fireplace.

I’ve been thinking of doing this also, to put the tv above the fireplace but improve the look when the tv is off. I wonder why it doesn’t seem to be very popular. Is the picture quality bad? I wonder how the ambient light effects it. Hmm.

You can also do art instead of a mirror:

https://www.samsung.com/us/televisions-home-theater/tvs/the-frame/highlights/

Frame is a good idea and it looks like a cool concept, but after a nightmare dealing with Samsung trying to purchase a $3300 fridge back in Feb using their financing which they couldn’t seem to connect with (told me to contact my bank to approve the transaction... which was them), having them cancel the extended warranty I purchased without me asking and being told the only resolution was to start over and order the fridge and warranty at new higher prices, and spending 15-20 hours on the phone consistently being passed from one customer service rep to another (sometimes 8x on one call) with each rep making no attempt to resolve the issue, and having their shipper try to schedule delivery for “tomorrow” only for the shipper to realize they haven’t received the product yet from Samsung so it definitely would not be delivered, I am done with them and cancelled the order last week. Bought a Bosch - equally expensive, great reviews, arrives next week and at least the transaction went smoothly! I know this is supposed to be about TVs, sorry! Frame is a good idea but I can’t out of principle.

We toured a home built by our builder (custom) and they had this setup behind their bar in their walkout basement - it was very cool! Did not see the TV turned on, but it was definitely a wow! ($2m custom home, though)

I built a home in 2002 and put an alcove above a double-sided fireplace for a CRT TV. I mounted a two-way mirror, hinged on the side, above the fireplace. But it was adjacent to a large window and during the day, all you could see was the view out the window reflected in the mirror. It worked out well at night but this was in our master bath and we expected to be able to watch the morning news before heading out for the day. Fortunately, we could still watch the TV by swinging the mirror open but I wasn't proud of my cleverness!