I don't think it'd look too great.. I'd keep the white.
By replacing it with a black grill, I think it'd make the car look a lot cheaper because it would just remind me of those cars that are too cheap to have body-colored parts, so they have generic black parts instead. (like many cheap cars that have black mirrors only, or other black body panels no matter what the color of the car is.) You should feel lucky that GM actually put forth the effort to make everything body-colored on the Bonnevilles, to change that, would only make the car seem cheaper. Just my opinion.