I took care of a horse that had this on her head, it took the shape of a blaze, while she didnt have any markings at all. It was just quite a lot of white hairs mixed in her black noseline with a sharp border. So many you could see a shape of a blaze. I cant add a photo of her as I dont have it online anywhere. Might be the same spell on horse coats present there where something supposed to be white/spotted but for some reason couldnt express it. I dont know if that exists. I just figured might be same with your horse maybe having a tobiano pattern but not expressing it other than with few white hairs in the basecolor coat, as you said its like patches of white hair. If I owned your horse I sure would do an extended coat color test even if just for curiousity.

I had a chestnut overo mare who had the same white hairs in certain areas of her chestnut area. Her foal (a black colt with no markings except for a small marking on his nose) also had those same white hairs once his coat turned. Neither were roan as far as I'm aware. But your horse could have minimal roaning as someone above suggested.

Possibly rabicano, he's not loudly marked enough to be sabino, and roans don't just turn roan, they get roan hairs showing through from a week or so old. Some horses have white ticking (white hairs scattered throughout the coat).

Those are two very handsome horses