No "Blackness", "Microdynamixs" or "sibilance" in there.
"grainy A moderate texturing of reproduced sound. The sonic equivalent of grain in a photograph. Coarser than dry but finer than gritty."
Say what???
Sibilance - System/transducer(s) has a peak somewhere between 2500hz to 10000hz. In speaker systems, it can be caused by reflections and time alignment issues (ie you hear the output from each speaker on the same note out of sync). The latter is a much bigger problem on cars that are not the Mclaren F1 than any home system.
Black and Grainy - Take a good camera. Take a photo of something with blacks in the frame at ISO100 and then at ISO6400. See how clean the black is on the first setting, and how grainy it is on the latter? This is what people are describing. In a more direct sense, "blackness" means "very low noise floor," while "grainy" is "(a particular kind of) noisy." A very quiet system will not necessarily have "louder bass" or "detailed midrange/highs" as people think it does, it just has less noise that gets in the way.
Easy test: select a track, listen to it at the level you normally would, then hit pause; if you hear any electronic noise, that's grainy, maybe just not obvious. No noise means the noise floor is low enough. Gradually increase the volume. If you hear any noise at that point, then your system is just clean enough for the level you listen at with whatever headphones are plugged in (note that a very high sensitivity headphone might actually get more noise than a slightly lower sensitivity headphone). If you get to max setting with no noise, then at least with your headphone the system is very quiet.
Expensive test: get an eBay tube amp (the more dubious the better) and a Meier, Violectric, or JDS Labs amp, plus an ODAC RevB or Magni2 DAC, then run the same audio test. Cheap eBay tube amp will likely show you what "grainy" is like. Note though that I myself got a cheap hybrid amp from Amazon - a Pangea-modded version of some cheap amp from China - and much of the noise is not only way beyond my hearing levels, but also coming more from my smartphone (which I use as a music server) if not set to Airplane Mode. Needless to say, while I would not have bought it when it was sold for $250, I was floored when a then-$100 hybrid amp wasn't doing as badly as I thought it would, or at least, not on the high impedance, high enough sensitivity HD600.
I agree that most of these terms are made up to describe a subjective sound. By definition, as it is subjective, it will mean different things to different people. But anyway this is what I've known these terms to mean.
Black - overpowering and pervasive bass frequency emphasis, the opposite of bright.
This is a perfect example of the problem of all these descriptions - bass emphasis vs treble and midrange is descried as "dark," and some assume that "black" lies farther to the left of that spectrum. It does
not. "Blackness" refers to a black background, and if on a spectrum, the other end is "noisy" (more commonly used is "grainy"). Look up a photo of someone in a tux at ISO100 with proper lighting (even with a speedlight) vs ISO6400.