I've only seen unpainted chrome in that area. The red is correct, though you'll see many cars have that gone, as it is anodized and tends to fade irregularly. I steel-wooled my red off my car in the mid-'80s.
I've owned my current '66 500 since 1980, and have owned 16 '66 Coronet 500s over the years. Not one had the black.
It is possible you have an oddball factory bit there...possible.
That black looks great! If the whole part is in that good of shape, I'd leave it and install that part on your car.[/QUOT