My new apple watch 8 screen remains black even when wrist raised. I must press Digital Crown to light up screen.

Make Sure Water Lock is not turned on - blue droplet at top of screen. If so press and hold the crown to turn it off

You should be able to tap the screen to turn it on too but...

On the Watch app on your iPhone, tap on My Watch

then Display & Brightness

Check that Wake on Wrist Raise and Wake on Crown Rotation are turned on

Also Check in Accessibility that no custom settings prevent it

Ah yes that one too!