what version of ubuntu are you running? I've seen this behavior with the old 'gnome-screensaver' (which I think ubuntu hasn't used for quite a while), but with

what version of ubuntu are you running? I've seen this behavior with the old 'gnome-screensaver' (which I think ubuntu hasn't used for quite a while), but with the current gnome-shell lock screen it behaves properly and doesn't black out the screen when I open my laptop.