I am endlessly fascinated by the term and concept of the blacker background.
I can't tell you the number of times we've made one technical change or another and hear the background noise drop away like black ink on velvet. But is that what's really happening?
I sincerely doubt I can hear the differences between a few dB of background noise, yet the sensation of a blacker background is both repeatable and obvious in A/B testing.
This is an area I would someday like to spend some time researching to find the cause. I don't believe it's actually lower background noise per se, but rather, I am guessing it's a lowering of attached noise and grunge riding along with the signal.
Whatever it turns out to be, it would most likely be something other than what our words try to describe.