Apple Watch turns black

It may be a hardware problem. Your best option may be to make an appointment at the Genius Bar in an Apple Store near you and let them look at it:

Genius Bar Reservation and Apple Support Options - Apple

You can try the steps in:

If your Apple Watch won't charge or it won't turn on - Apple Support

For repair & service:

watch Repair - Official Apple Support

watch Service Pricing - Apple Support

Your Apple Watch Series 8 can have Always On display, but it is optional in the settings. You can look in either of 2 places,

• In the Watch app on your iPhone check Display & Brightness/ Always On

-or-

• On your Apple Watch check Settings / Display & Brightness/ Always On