Maybe its not truely breed standard. An extra bloodline may have been thrown into the mix giving it that coloring?????? I honestly have no idea
Here is a bit of something from the Friesian Horse association website. Q. What colors do Friesians come in? A. The only color a studbook-registered Friesian comes in is black, however this may range from very dark brown or black-bay to true black. Many Friesians appear black bay when their coats are shedding or when they have become sun or sweat bleached. Selective breeding minimizes white markings and the only white marking allowed on a studbook-registered horse is a small star.
I'm going to say that we're using an Arab and a Morgan(I randomly picked two breeds). You breed the Arab to the Morgan and get a baby. Then take that baby and breed it to an different Arab. Take the baby of that group and breed it to a different Arab. I think that's how you would get a 3/4 of one breed and 1/4 of the other. So complicated.