My laptop (GeForce GTX 860M) display goes black at random times, while the external display (if plugged in) remains visible. The blanking is not limited to contexts when the external display is plugged in. When on battery, or plugged in, in all those contexts the laptop display very predictably goes black after short use of the display. The only context when it remained visible is when there is no application launched on the laptop display, just the desktop picture visible.
I have described a lot of detail in this Ask Ubuntu post: https://askubuntu.com/questions/1024576/laptop-screen-randomly-switches-off-ubuntu-16-04-4-lts-external-display-attach
Today I have switched to the NVIDIA binary driver and it appears that no blanking occours anymore. At least in the typical scenarios that used to cause the blanking, for example full-screen YouTube video.
My guess is that the built-in display mode "brightness turned a bit down" that kicks in in some power-saving scenarios, is not properly implemented and causes the display go black.