Is there a way of getting this watch to turn completely dark when it's not being used? Like if you cover the screen with your palm on an apple watch?
Yes, but it would require making your own custom watchface that only shows when you turn your wrist, similar to how the amoled models behave. No way to do the palm cover, it doesn't have a touch screen.
When I had a 255 I made a watchface that displayed minimal info when it's not being used and then extra when gesture activates it.