That's strange as on my 2008 assembled in Mexico 50 inch Panasonic plasma, the black have always been black, as black as the screen frame.
Perhaps my black is blacker than yours? My mid/upper level Panasonic plasma had grayish bars for non-full screen content, which was quite apparent for darkish material when in somewhat dark room. And yes, contrast was reduced.
Plasma was nice but recent OLED is better, at least for content I watch
