"Black" Andalusian showing grey hairs?

My school has a donated "black" andalusian mare around 6 years old "appraised" at $42,000 due to her color. The mare is green broke, barely, and I have done most of her training this year with the assistance of my instructors. But here is my question, can a pure black andalusian, as they have her listed, grey out? The mare is showing a plethora of grey hairs about her body which seems odd. I have already tried Google to no avail, any answers? Oh, and the owner who donated her had her registered as bay with IALHA, but when they donated her to the school (tax write-off) they wrote her color as black.