Note that some black is expected on all tail feathers for female McKays Bunting and that hatch-year females are "highly variable" in regard to amount of black. In all cases, it a lot less black than one would find on a Snow Bunting.
I believe pure Snow Bunting can be eliminated based on current information and that the case for hybrid is pretty weak.
Folks are seeing the bunting flock this morning. I saw the pale bird in flight well enough to confirm Tim's observations from yesterday regarding his description of the tail. It looks very white at the resolutions one gets from a moving bird at a distance through binoculars.