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

room, noise, appliances, AC, windows, etc....think BIGGER than just the stereo and AC power...few do..

double core door, weatherseal, etc...

and a good spl meter will help you establish a baseline noise floor, so few audiophiles own or use them...

Noise floor down. Power, components, cables, vibration control, room,.......mood, and no lights.

G

@tomic601 what spl meter do you use? I like that suggestion.

Overall, I think it has been the dedicated line that helped the most so far. Unfortunately, there is a refrigerator 25 ft from the listening position that ain't going anywhere... 

+1
"ebmEverything helps."

It is the result of a low noise floor. Everything matters, from low noise power supplied to your system, to draining internal noise from components, to how components work with the rack where they are placed, and isolating components from outside noise and vibration.

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.