Can I glue mirror tiles over an old mirror to hide the black spots?

there is resilver material you can buy but if you dont want to go that route, try a wide trim of some type around the mirror if that's where the black spots are. usually the spots are from where the back of the mirror has the silver paint coming off. http://codeand.me/wp-content/uploads/2019/01/mirror-over-sofa-above-couch-mirrors-and-she-decided-to-make-a-similar-hanging-behind-hang-set-of-your.jpg

Yes, with a really strong adhesive. You can use liquid nails with a caulking gun to make it easy, or E-6000 is a strong adhesive as well. You can find these at the hardwood store or craft store.

Hello there,

Yes you can either as a trim or all over. You can also use small edging ceramic tiles - Mosaics....... Or Self adhesive wallpaper border etc.......

Yes as long as you cover the mirror completely to prevent a reflection.