Black comes in various shades. choose a matching black for that part of the mirror. Duplicolor should have a match, and you have the convenience of using it as soon as you get home from the auto parts store. Use a plastic adhesion promoter if you're painting directly on plastic.
You did a beautiful job on that mirror! Unfortunately, all the time, work, different products needed on that project would be way too much for me… ☹️ (not to mention I have TWO side view mirrors). Maybe I’m wrong, but it seems to me that it would cost just as much to purchase all that stuff as it would to pay a professional to do it. - And I could save myself all the prep, work, and cleanup. IMO
You may be right. It makes a difference if you already have all those items on hand. We've been restoring cars for a number of years and have the professional gun on hand, for example. Some hobbyists may have some of the materials and equipment in their shop already. Either way, it's a small project for novice and expert alike. Thanks for watching!