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

@yermajesty, Shielded XLR like the Mogami Gold 2534 can help IF you really have RFI/EMI problem, otherwise the 6db gain (or so) of XLR versus RCA will just raise your noise floor by 6db... I experienced the thing and got back to a good shielded RCA.

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.