"Grainy" sound vs "black" sound - what does it mean?

Grainy typically means certain narrow bands of frequencies are peaking (either due to ringing issues or more often due to intrinsic funky frequency response) and so the end resulting sound is not smooth, because of that narrow frequency emphasis. To simplify this, think of frequency response as a piece of cloth, and peaks as little pieces of sand and grit caught up in the cloth, the grain would be what you feel when you rub that cloth against your skin, vs a grainless FR which would feel like a smooth cloth.

But it's not that simple, since music itself tends to be far from perfect, and due to recording / mastering flaws can have intrinsic peaks that show up as nasty grain/tizz/shrilly effects even out of good smooth headphones.

Also resolution is a factor, a smooth FR headphone that is not very resolving will be extremely forgiving, even to poor recordings outlined above. But it will also lack in resolution so you will miss details. To continue using the terrible analogy I started in the first part, think of low resolution headphones as a knit wool fabric, and high resolution cans as a finely woven piece of silk, both will feel smooth but one will be more revealing and transparent to details (as well as grits/grains) than the other.

Black background is something unrelated. That usually concerns how much harmonic distortion there is.

I've always taken it to mean the level of detail is such that you can hear the 'grain' of the recording. Borrowing from the film world "Bluray is so good you can see the grain of the film!"

Where as absence of grain might indicate a softness that tends to smooth out small flaws.

This is my interpretation anyway. For all I know peanut butter to me tastes completely different than it does to you. :)
 

Heya,

I find Grados (prestige line) to sound grainy. Grainy describes that the sound is raw, not smoothed over or relaxed, but rather, very forceful, full of attack, rough.

I find Hifiman planar magnetics, when properly powered, to have black backgrounds. They're transparent, no noise, very fast, very detailed, very clear, and there's definite separation to sounds.

Very best,

I have a comparison (just IMHO):

Imagine the sound of a clarinet or a saxophone and the performer playing  only with his (or her) lips surrounding the mouthpiece.

Then imagine the same sound but the performer now bits with his (or her) teeths the reed of the mouthpiece of the clarinet.

When the reed vibrates free (only gently surrounded by the lips) it produces a smooth and clean sound, but when the player bits it (or press the lips more than gentle) then starts to produce a "buzzing" sound.  When controlled, this "dirty" sound becomes nice and adds a "sexy" signature to the sax (by these days is a commercial cliché for sexy 9-1/2 weeks kind of things).

My analogy is not perfect but I think can illustrate the kind of differences between more or less grainy sounds.

In the world of sounds, human voices (more in the baritone register),  reed instruments (clarinet, saxophone, oboe, bassoon, etc) and metal winds (trumpets use the lips as vibrating "reeds") are more prone to sound "grainy" but sometimes the recording itself can emphasize this tendency.

The above is only a reference and, of course, just an oppinion.