How do I keep my bathroom mirror from turning black on the edge?

Any mirror will do this in time, especially if they are exposed to moisture day after day. Here are a bunch of tutorials on how to frame it. I did all three of my bathroom mirrors, and love them, and it's so easy to do!

https://www.youtube.com/results?search_query=frame+mirror+in+bathroom

It's most likely that the very thin layer of pure elemental silver ((which is created by a chemical reaction that occurs directly on the surface of the glass)) is deteriorating. If this is a valuable mirror--at least to you-- you can have it re-silvered. Not inexpensive, but when done by an expert it will last many years.Your best, most affordable option is to purchase a new bathroom mirror. Even the finest mirrors eventually deteriorate.

Moisture got behind the silvering and breaking it down. All you can do is to frame the mirror to hide the damage.

https://www.hometalk.com/search/posts?filter=frame%20bathroom%20mirror

There's nothing you can really do except either cover it up with framing or replace the mirror all together.

Hi Wilton, caused by moisture, you can hide it by adding a frame or decal to cover the dark areas, You could even use glass paint to paint over the spots, hope this helps you out,

https://www.wikihow.com/Paint-a-Mirror#:~:text=Acrylic%20and%20water%2Dbased%20enamel,should%20not%20be%20an%20issue.

Okay, maybe I will look into the paint. A frame would be nice but I am not a carpenter and don't have those skills. Thanks for your advice. Wilton