Whether you want to think of such a dog as being a black with tan underparts or markings, or if you call it a tan dog with black saddle markings such as saddle or mask. (siriusdog.com)
The Landseer Newf is a white dog, with a black head, black on the rump extending onto the tail, and an evenly marked black saddle over the back. (canismajor.com)