That only helps when the device doesn't have an LED that's inside an open casing and so ends up visibly shining from every hole in the device and/or through the thin plastic itself.

these are the worst, or when they are on a button like my monitor

I've been sticking these things on everything. Not this particular brand either, but I doubt there's much difference between brands.

It allows enoughlight through so you can see what's going on but not enough that will illuminate the room or distract you just by being there.

I never thought to look for that before! Amazing. I currently have a lot of black electrical tape over everything.