I would vote grey just because the mother is grey. I know it is likely 50/50 chance, but I find it tends to be passed on heavily. But you may not see it expressed for years. My instructors 5 year old Andalusian stud is a dun (expresses all the dun factors, despite dun not being a legitimate color in the Stud Book, I know), is a grey, like his mother is, but has only greyed out in his face. His half sister, about the same age, is pure white like his mother is. Sire is a beautiful bay. The rest of him is still bronzey and black. It has been suggested by color experts (UC Davis) that he may have a pearl gene (which could explain the distinct lack of grey expressed by such a late age) but no guarantees until he is color tested.
The only thing "special" about Kiger Mustangs is the fact that they have been isolated and undiscovered until the 1970's when the BLM started using helicopters in their round ups. Their genes have a throw back to Iberian blood, which means they have various breeds of horses leading to them that came from the Iberian peninsula (Sorraia, Andalusian, Lusitano, and many more, there are up to 18 breeds of horse considered to be 'Iberian'), that is it. They are not the only ones. In fact, there are "island ponies" in IBX, North Carolina that have the same throw back.
And, AND, if roan, particularly black/blue roan, is so rare, why can I go to various horse sale websites and pull up tons, TONS, of Quarter Horses, Tennessee Walking Horses, Missouri Fox Trotters, American Standardbreds, etc for sale? Blue roan is so dang popular there are barns that specialize in breeding true blue roans, in all of the breeds I just mentioned.
It is just a roan gene on a black base. It is not that hard to come by.
Don't ever, EVER claim to know more than anybody. That proves that you have stopped learning, and therefore have learned and know less than most.
Apologies to everyone for the rant. I'm frustrated this evening.