Hi, I was having the same problem after installing the proprietary nvidia drivers (nvidia-driver 390.87-8~deb9u1 from backports) with light-locker and lightdm. Locking the screen (I use xflock4 from Xfce but any other locking command had the same effect) would send me to a black screen with a fast blinking cursor. Using Ctrl+Alt+F1...F7 to change vtty would not work (vtty would appear for a very short time and I was sent back to the blinking cursor).
The solution I found was simply to generate a /etc/X11/xorg.conf file using 'sudo nvidia-xconfig'. The docs at https://wiki.debian.org/NvidiaGraphicsDrivers suggests it is no longer needed to run nvidia-xconfig, but for me it was. After running it, the screen locking mechanism immediately started working correctly. Maybe the wiki should recommend running sudo nvidia-xconfig anyway to avoid such problems? It would be even better if the nvidia-driver package's postinst script did it automatically. I will report this to nvidia-driver maintainers.
