It may be a dorsal, but I don;t think it is due to dun. Buckskin can also have dorsals and darker points. This is my filly from last year...perlino, darker points and dorsal stripe. (no dun)
But when a buckskin have Dorsals and darker points doesn't that make them a Dun??? Maybe not but thats what I always thought
First of all congrats on a gorgeous baby!!! It could be a dorsal or like Andrea said it could just be countershading, its really hard to tell with a photo especially when the horse isnt clipped. As for dun characteristics, the way I understand it is when it is a buckskin with a dorsal that makes it a buckskin dun because if there is a dorsal stripe it just means that they carry the dun gene, a true dun should also have at least some shoulder barring and preferably leg bars too. Just how I understood it, may not be right but its my take on it, maybe one of the genetic gurus on here will say more!