Poseidon is right on...I own a buckskin and she will sometimes have what appears to be a dorsal stripe, sometimes not. But it's actually countershading, she lacks any of the other "dun" characteristics.
Like Po mentioned, buckskin is a Bay with the "cream" gene...so basically a bay whose brown-colored parts are now a cream color of some shade...but the black remains black, which is why you see the black-trimmed ears, black legs, and black mane and tail still. Buckskins can come in all sorts of shades too...mine is more a "buttermilk buckskin" color, for example, here's a photo of her. And here's a website that does a good job explaining too http://en.wikipedia.org/wiki/Buckskin_(horse