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

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.

pragmasi gets closer than most. There's at least four main types or categories of noise.

First and most obvious is room noise. Fans, heaters, airplanes, all that stuff. This is the one out of all the noises you might actually be able to quantify with a meter. It is also the least relevant to obtaining a black background.

Then there's the room noise that is created when playing your music causes the room and everything within it to vibrate and resonate, and all that chaos of vibration gets fed back into the air as noise. Which if the room is treated just right we like it and call it ambiance. Otherwise we call it slap echo or whatever. Either way, again, very little to do with black background.

Then there's system noise or what most call S/N ratio. People sucked into the digital con game tend to love measuring stuff and digital does measure really low S/N. But then we have no way of hearing it without hooking it up to all the other stuff in the system, which is in a room, and the much worse S/N of all of that drowns out the digital number, which was only there to look good on paper in the first place. In any case, and as usual, go f, oops sorry that was Tom Cruise in Tropic Thunder. As usual, this one is not all that either. Although it probably is a lot less unimportant than the others, which are almost completely unimportant. 

In terms of obtaining a black background the one that rules above them all is the one no one has figured out how to measure but everyone easily hears no problem, and that is the purity of the signal and the ability of the system to stop as fast as it starts.  

Most audiophiles shoot themselves in the foot here. They think, fast is good! Gotta have fast! It is real easy to hear when something is fast, it takes off instantly. This catches your attention and you buy it, only later to tire of it because this kind of fast tends to be more etch, hype, grain, not true speed, and you catch onto this, get fatigue, upgrade. Round and round. 

True fast is the sound stops just as fast as it starts. 

The black background we want to hear, it is the acoustic of the recording venue. Being typically either a well treated recording studio or a large concert hall, either way the room itself makes no sound of its own. The sound we are looking for is extremely low level. But here is the key: low level relative to the music signal. 

This is why the room noise matters, but not that much. And the system noise matters, but not that much. Because when you are listening, whether to a recording or a live event, you are not some meter measuring out absolutes. You are a human being making comparisons. It is the sound of the room relative to the sound of the performance that tells you where you are. When you get that balance right in your system, there is your black background.

This is why springs work so well. Without them you play music, it excites the whole room. Your speakers, amp, source, are all part of the room. They all vibrate, and this noise feeds right back into the signal and smears those subtle acoustic cues and you lose that black background. Then you add springs, now the room still vibrates but no longer feeds back into the system. Signal purity improves and you get a black background. Speakers, free to move on their own, are no longer anchored to the chaotically vibrating room, and you get your black background.

Or you add a good power cord, interconnect, or speaker cable. Signal purity is the hardest one of all. When the signal is as unmolested as possible then you get your black background. 

Proof positive of all this is in my room. The room noise is very low and so everyone easily hears the system noise. When turned up to satisfying volume level the system noise is easy to hear from the sweet spot. But that doesn't matter because when the needle drops the groove noise is all you hear. But that doesn't matter either because when the music starts then suddenly, amazingly, there it is, floating in the room, in a background big and black as a cave. 

Oh and this thing stops so fast with signal so pure you can hear that black background acoustic even when the music gets good and loud and complex. Freaking amazing! Because even though I said those other things don't matter all that much, they do matter some, and you want it all you just have to do it all. 

Just use your ears. Not a meter.

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.

there are 3 sources of noise:

Mechanical noise coming from unwanted vibrations and resonance...

Electrical noise floor level of the HOUSE not only from the room and gear...

Acoustical treatment and controls to help listening what has been gained by controls of the mechanical and electrical dimensions.. But the room non isolated will be also a source of noise...