Justubes, there are in fact two different issues here, but my understanding is that "dark," what you call "first dark" refers to something's tonal attributes, as in: The current Levinson amps have a dark sound.
The other issue, noise floor, or background noise, what you call "second dark" I've always heard referred to as "black" or "black background", never "dark." As in: The sound emerged from a totally black background. I've never heard the term "dark" used in this context.
In the first case, a range or degree is implied (dark, darker, very dark ;--)
The second case implies an absolute -- absolute silence. The sound never emerges from a "dark gray" background ;--)
.
