I have an arabian colt (1/4 hackney) who is bay. He has a black stripe down his back, and my trainer mentioned he has dun markings. Is this normal? Have you seen it before? I will post pictures soon. His mother was mahogany/seal bay and also had these markings, and his father was white grey.
Since he's Arabian, he can't be a genetically a dun, hackneys are not dun either.
So this is most defiantly a case of counter shading, or in other words just the hair gets dark down the back and resembles a dorsal stripe.