“Black background” — What really contributes to this phenomenon?

Filtering the AC noise is what will provide a black background.  You can do this with adding an AC filter choke to most components.

Happy Listening

I have to admit I’m a bit bemused with focus on noise from the mains... I’m not saying it’s not a problem, it’s just never been a problem for me... can someone explain what mains noise sounds like in a way I might understand, like bangs, pops, thuds, hissing, hum, buzzing?

Mains noise you can hear sounds like a hum at 50 or 60 hz but there is more to it than that , cheap switch mode power supplies, powerline ethernet adapters LEDs especially ones you can dim in your home all introduce additional noise (most of which is at frequencies you can’t hear) and DC offset. This and groundloops destroy your soundstage, tibre and microdynamics.

some excellent advice already in this thread.

Higher end components can deal with noisy mains better , but you can still improve by paying  attention to details. Once you have this sorted then you can look for even quieter (blacker) backgrounds by moving on to vibration control.

well designed cabling (which doesn’t have to cost your left nut) and well dressed can also really help but this is about protection of the system from rfi/emi mostly

I’m assuming you have already given thought to speaker and room interactions.

take care off all of that you will get great sound.

@yermajesty, Shielded XLR like the Mogami Gold 2534 can help IF you really have RFI/EMI problem, otherwise the 6db gain (or so) of XLR versus RCA will just raise your noise floor by 6db... I experienced the thing and got back to a good shielded RCA.

Low noise.

And the most significant factor in getting this is using specially manufactured low noise transistors at the VAS stage of the amplifier.

Of secondary importance is to have very well shielded cables, maybe a ground lifter and no ground loops.