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

Low noise.

And the most significant factor in getting this is using specially manufactured low noise transistors at the VAS stage of the amplifier.

Of secondary importance is to have very well shielded cables, maybe a ground lifter and no ground loops.

The resolution of the components and cables matters a lot too. Not only to reduce noise, but to being transparent enough to discern the micro detail and air in the recording. Getting the black background and not wasting it with lossy signal path is a real investment! 

@bigkidz adding an ac filter choke? Please elaborate. 

Setting up your entire system with low noise components and power conditioning and dedicated line feeding your system will guarantee that blackness you want.

Ever used your dB meter in a 'quiet room'?

Nothing running.  Shut off EVERYTHING.

"Black" is subjective.

Even in the 'quietest' of spaces, you're still breathing and your heartbeat is occurring with annoying regularity....

...and that blood running in your veins is 'hissing' away....*sigh*

Best you can hope for is a darkish gray....;)