Plasma's have a different Black Or low Y luminous set, where as the contrasting on Black and white are grey actually, so when an image calls for 'off' or black contrasting like on a black coat in daylight the coat actually appears grey and lighter than it would be on a LCD or CRT, the reason is the pixles can not turn off completely, it doesn't matter about the screen back film color. Plasmas are just that way, it is more difficult for plasmas to show black contrasting and they use more energy because of this as the pixles cannot be totally off or totally black. However quality of different plasmas is huge and diverse even on the same model made on a different day.
I personally don't like plasmas for this reason, they may have high definition, but it there is no high definition or HDTV in your area it is pointless to get one, I know I am not going to get one when only 7 channles are HD.