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

Power? Sources? Turntable? Amps? Cables? Tweaks?  Vibration control? Any of these in particular?

No. Not really. It's all of them, and more.

Oh and, fixed it for you.

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.