After waiting a for an update to my Linux Kernel from the Fedora repos (I don't fancy building my own just yet) the open source drivers for for my Radeon RX460 were installed successfully.
This seemed to sort out some of the issues I was having, the visual artifacts at higher resolutions particularly, but didn't sort the issue of the screen going blank for periods of time. Which very quickly became incredibly irritating.
After convincing myself the drivers were ok (discussions of Fedora support forums), I started to wonder about overheating. Given the blackout seemed to happen most when playing video, but that shouldn't cause any real effort from graphics card like the Radeon RX460. Clutching at straws a bit with that idea.
Sure enough, xsensors show there's no clear heat issue:
Back to the drawing board then.
More reading then. Started to wonder if the Benq BL2710 monitor was the issue, so started reading more reviews of the panel to see if any reviewers had a similar problem when testing.
I eventually found comments in a about the power save feature of the BL2710. Apparently it has a sensor on the front that attempts to detect if there is someone in front of it and shuts of the display if there isn't for a period of time.
This is indicated by flashing of red LEDs at the bottom of the monitor, something I noticed happening when I experienced the blackout issue.
Sure enough, after exploring the menus I found the option to turn off (ECO -> ECO Sensor -> Off) and did so. Since then everything has been working without problem.
I'm surprised I didn't find any other articles on this black screen "issue" with this Benq, it must affect other users too.
Anyway, all up and running at full juicy 2560 x 1440 resolution now :)