Would it just be pitch black? Or have streaks and stars like you rubbed your eyes for too long?
Ghostly misty white? Like in a fog?
Would it be a flat colour or shade or something that moves?
Perhaps some sort of specific background like a Tv static? Or a room/building you like similar to what some vr spaces do but recreated with splotches of colours maybe?
Or something more exotic like a constantly changing rainbow?
Would like to hear what you’d choose and why that specifically
The assumption would also be that you can’t really change it later, the choice and all the variables would be locked in, barring some minor variation perhaps ( brighter/darker or more/less vibrant for example)
In this case, should you manage to learn echolocation, what outline would the items you perceive be? Against what colour background?
I think they would be fuzzy rather than sharp and a grayscale with a violet tinge, a purple-ish gray.
(Though since I’ve spent so many years with sight, correctable to near 20/20, and reasonably good color perception, if it was a place I’d been I think I would “see” it close to the colors I had seen it.)