No true black appears to be available

Try this:

Turn the monitor on. Make a background picture of a black and white checkerboard pattern.

Press and hold the 1, up and down buttons. A small icon will eventually show on the display and the monitor will do a white ballance.

It can be hard to get the monitor to do this sometimes. Just keep trying until it works.

Note, you should open the monitor and look at the caps in the power circuit. They may need to be replaced. Better now than later.