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

A thought on Noise (the nemesis of silence between notes) starts at the front of the gain stage chain (assuming that all the stages are working right).  It has a simple equation from information theory,

 Noise = f + 1/f  + 1/f^2 + 1/f^3 . . . .

So look to the beginning of the chain.

Resolution in audio focuses on the distortion of the timbre of the sounds to which you are listening.  Noise between the notes is the biggest problem, as I have written elsewhere, it is a cascading effect, most pernicious in its nature, one hard to combat, and completely disruptive of the illusion of reproduced music. 

Hello,
It all matters, but what is the priority? Power, Power, and Power! It starts from the fuse box. Besides removing things off of the line to your system you have to control the ground. I know MC has done this years ago. OCD Hifi on YouTube has done this. If it didn’t matter in a huge way they would not have done it. It’s not cheep done right if you have to go a large distance or through a lot of complicated walls and floors. The last is if you cannot control the power coming to the room the you need devices. Puritan is the best I have tried. Also it lets you install a separate ground to handle those issues. Puritan PSM156 will filter out the DC garbage on your line and also filter the over the air garbage from cell phones, WIFI, and Bluetooth getting into the power. Most will “condition” and protect the power to the components. Where they lack is what and how they filter. Despite running four 8’ silver coated copper ground rods into the ground for a dedicated ground system the Puritan still helped bring the noise to ZERO. This is how you get the biggest start to a black noise floor. I bought my Puritan PSM156 at 
https://holmaudio.com/
If you are in the Chicagoland area you can try before you buy. No risk plus it will be broken in so you will know how good it is immediately. 

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.