How do I stop my external monitor from occasionally blacking out?
Does it sometimes go completely black and you can not seem to get it turned on again?
Does the monitor have its own menu button on the monitor itself to change monitor settings? 


I aks because sometimes the monitor has its own sleep mode independent from the operating system.

Check the monitor-menu and check what settings are there and try find a manual - either online or the one made out of paper that came with your monitor.