Decrease screen brightness to dark

Hello,

I'm using Windows 8.1 and I managed several times to darken my screen just by decreasing brightness of my screen to absolutely minimum (on my keyboard by pressing buttons "Fn" and "left arrow"). No just decrease brightness, but to completely turn off the screen.
But today I realized, that after Windows update I can't do that and I'm confused.
Maybe the previous functionality is because I had enabled option "Auto-adjust brightness", so the screen was darken so much (in particular level), so that when I decrease brightness to minimum, it was darken so much in the minimum brightness level and looked like the screen is black, but it wasn't.

Is it even possible, that before the update I could have done such a thing? Does Windows allow to decrease brightness to (almost) black?

I have notebook Acer Aspire and graphic card drivers are up to date.

Thanks a lot in advance.