What's the best way to block out area of mirror that doesn't have a monitor behind it?

[fribse]

Ahh, ok, then I know exactly what it is :-)
I got some oracal651 as suggested by @Hubschra01 , but that was slightly more transparent than the one I already had. So I’m going to stick with the cheap stuff I already had, and return the oracal651.
@Goldjunge_Chriz is it correct that I can foil the back of the mirror and then cut through the foil without ruining the ‘mirror foil’ as that is applied to the front of the glass in your variant?

[fribse]

I sent the oracal back (very expensive stuff) as I could get much cheaper and darker stuff from our local hardware store.
So I got the back covered with it, and then cut holes in it for the LED lights and monitor, it seems to work very well.