When watching a video on my laptop what causes it to go to a black screen where

that sounds like a screensaver or power saver option.

For power options:
open your start menu and type in power, select power options. select to change when to turn off the display (left hand side) and set it to never.

For screensaver:
right click desktop and select personalise, select screensaver and change it to "none"

hope this helps