Plasma displays do not modulate light with voltage like an LCD does. There is only one light intensity and to create gray scale the panel must use a form of pulse-width-modulation using subfields.
A subfield is an alotted time period to emit light within a frame period. These time periods are weighted either in a binary scheme or contiguous.
For example, Panasonic uses Binary I believe. Therefore there are 8 weighted time periods to emit light per frame.
The panel then can selectively use combinations of these subfields to create 256 gray levels per sub-pixel (R,G,B)
As for black level, it would take 5 pages to properly explain but in short Pioneer is king thanks to the use of CLEAR driving in combination with a very high gamma electron emitting layer on both the phosphor and sustain electrodes. This makes it very easy to reset and prime the pixels with very little unwanted light emission