You can also get sheets of hundreds of small little black dot stickers that are perfect to use on LEDs. Some of them are thin enough to let some light through an individual one, so you can choose to dim or entirely block by adding one or 2-3...

You can get a lot of toys but if your or your significant others' cosmetic habits include black nail polish why not use it?

You can’t remove the nail polish (without likely damaging the base surface) when you want to sell or gift the device to someone else, and it doesn’t look good on non-black devices. (Even on black devices you might not like its glossiness.) Black nail polish also completely blacks out the light, whereas one might only want to reduce the brightness (which the product I linked to supports).

I don’t see myself selling a printer; most likely bring it to recycling when it dies in 10 years.

As for gifting, if they object to my aesthetic choices that can always wait for someone else to gift them one.

Well, you asked for reasons. They may not apply to you specifically.

Painting the printer in Vantablack is also an acceptable answer.

That might be the one thing more toxic than toner powder particles.