Somatic. Means she has the DNA of only 1 horse, but her bay gene was deactivated from expressing in that patch. If you did a gene test in her bay area, and a gene test in her black area they would be genetically identical.
Probably dominant white == Sequoyah was definately not a gray. In the Spanish Mustang registry horses colored like Seqouyah are often called Sequoyah Blues. (Rare colored horses! POST AWAY!! - Page 16)