Apple Watch screen turning black

It could be that you have selected the option that always on is deactivated. So it changes to dark.

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How to enable or disable the always-on display on the Apple Watch:

Go to the settings on the Apple Watch.

Select the "Display and brightness" section.

The option is called "Always on" in watchOS. Tap on the entry to turn the "Always on" function on or off.

Under "Show complications", "Show averages" and "Show apps", you can select what should be displayed on the "Always on" screen. You can also deactivate the display for individual apps.

you also can have a look in Fokus. Here you can set up how the watch will be displayed.

Are you trying to use always on display? https://support.apple.com/en-us/HT210428

Check to see if you have theatre mode turned on in the control centre;

Have you tried to restore your watch?

On which iOS version is your system on?