We installed a large flat panel TV over our fireplace, and we love it. We rant he cords under a piece of crown moulding and down to a media cabinet in the corner of the room. The crown moulding is easily removable with screws if we need to access cords in the future. Good luck!

I absolutely agree with joots above. I think that big black box is an eyesore. I would put it over your fireplace as a last resort.
You have to hide the cables as the person above figured out, in some way or another. The wires and boxes need to be out of view or it will look awful.

A fireplace is soothing, cozy, the exact opposite of a TV. I would discourage you from doing this. Find another spot for TV.

The walls would be pre-wired prior to hanging the TV bracket and outlet.

New flat screen TVs now come with a mirror as the screen in the off position. Great frames give a TV a very finished look when not in use. Or you can mount a piece of art over it as well....