"I would never recommend test breedings"
Well no. Especially for geldings, LOL.
You know, it's possible that the 'fading black' thing is defined a little different by the researchers than by normal people. That might account for why it doesn't seem rare to us yet researchers say it is.
But I've never found anything that specifically indicates how they test for fading or non fading.
Well, you could just stick them out in the sun to figure out if they fade or not..
But I personally don't think it'd really be a "fading gene". Some people have their own hair fade in the sun. Some don't. Some summers my hair goes really blonde, some summers it stays dark blonde.