There is a genetic difference between a fading black and a true black however all colours fade as a result of exposure to sunlight so it can be difficult to tell without a genetic test.
What exactly do you think the genetic difference is? Black horses can only be EE or Ee, and that doesn't affect their appearance at all.
"True black" is just words. It's used to describe a black horse that doesn't fade. It's not genetic and it's not scientific. Same with "fading black."
You cannot genetically determine the difference between a "true black" and a "fading black."
Like you said, all colors can fade with sun damage. It's especially obvious on black hair - as in blacks, bays, browns, buckskins, etc. You don't hear anybody talking about a "true buckskin" or a "true bay." That's because it's simply a descriptive term.
I dislike the term "true
black," because it makes it sound like fading blacks are not actually blacks - which couldn't be further from the truth. I think the word "true" is what has caused some people to think that fading blacks are not black or are genetically different to blacks.