I think I know what your talking about?? So if I'm off base forgive me.
First, you can paint anything.
If these are natural black parts your talking about and best example I can give would be black door handles or unpainted mirrors.
If they are just faded, you can take a heat gun and heat up and the shine or blackness will come back.
Don't melt the plastic, go slow until you figure out the proper distance and the amount of time it takes.
I did every black piece on the rover before selling as they have a bad habit of the black plastic pieces blushing or turning white after about a year.
The whole truck took about and hour with all the black trim and bumpers.