@ DanielT :

Another possibility, I think:
(N 810, latest OS)

I have autolock (v. 0.3.1) installed.
When I cover the light sensor (shoving the nit into it's case) the tablet locks and the screen immediately goes dark.
Autolock is disabled(enabled) with the manual switch.
( Locking again manually immediately darkens the screen.
Occasionally the screen stays on during screen timeout, I do not know why.)