If you can feel these with your fingernail, they're probably there to stay. That said, I have quite often been able to improve black plastic trim with a heat gun. It is a scary endeavor the first couple of times you try it but once you get the hang of it not so much. You can use the heat gun on a lower heat setting until the plastic just starts to sweat. The end result can be quite amazing the color will be black again once it cools down. I learned this on a detailing forum a long, long time ago and it surprisingly works.