I guess what is the case(s) you are trying to detect?
Reasons the screen would be blank that I can think of
1 - You didn't put anything on the screen
Don't do that
But you could use the screen dump action to create a bmp of the current screen and verify that every pixel isn't black.
2 - sbengine crashed or has gone off the rails
You'd have to detect the crash and restart, but hopefully this never happens.
3 - hardware failure
Not sure what you can do here???
4 - power mode switched the lcd off
On linux this can be controlled and check by reading from a path in /sys/class/graphics/fb0