Accessible Feedback Master slide layers background reverts to a solid fill of #000000 and 20% transparency.

I was able to find the Articulate reference that described this, Storyline 360: Dialog Layers - Articulate Support. Apparently it's a normal behavior for dialog layers. When a dialog layer doesn't have a background color fill or has a background fill that isn't a color, Storyline 360 covers the course with a semi-transparent dark gray overlay. The fix is to use add your own overlay and adjust the transparency to the desired level.