Sony uses a potentiometer on their mainboard to adjust the Vcom (common voltage) setting. This is an additional voltage that must be applied to the display to compensate for a residual charge on the glass caused by addressing the display. The value for this is determined in the display factory and printed on it. During assembly they must adjust this setting manually.
However, adjusting the setting of the potentiometer yourself will only result in worse screen performance. You might be able to make black more black, but then your white will become darker.
The visual performance of the display is mainly determined by the quality of the E-ink material.