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

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?

I agree that eliminating noise from the system will achieve that 'black background' that many are looking for. I also think it's helpful to put noise into two different categories, one being interference and the other being random noise.
Interference is what most people think of and is under your control to a degree - so mains hum, radio frequencies etc. There are established strategies to deal with these and you can move equipment around or switch off sources of interference.
Random noise (think white noise) however is a product of the electronics within your equipment - so the designer has decided how much noise is acceptable and that is what you may or may not hear. This is mostly dictated by the resistances in the signal path e.g. if you have a pre-amplifier with a 10kΩ pot for a volume control then the absolute minimum amount of noise will be -114.9dBv if that is followed by a power amplifier with 30dB of gain then the noise floor can be no lower than -84.9dBv (as the noise is amplified along with the signal). So to get the lowest possible noise you need Hi-Fi equipment designed to operate with low internal resistances. For example if you replace the pot in the above example with a 500Ω attenuator then the noise values are -127.9dBv and -97.9dBv respectively. So if you want truly low noise you need to have equipment that is designed with exactly that in mind.

Proof positive of all this is in my room. The room noise is very low and so everyone easily hears the system noise.

That was my point... the system noise doesn't need audible either, it's the components that are creating it.