The Horse Colors Site

"The buckskin spot does indicate to me that the mare is probably a carrier of the cream dilution, especially since one of her parents was a dunalino. The cream gene has little effect on black pigment in a single dose, but will dilute any red hair. That is why her body has remained black, but the spot (probably originally sorrel) has been diluted. She wouldn't be a carrier of the (I assume she meant to say dun) dilution, or she would have been a grullo.  Random spotting on otherwise solid colored horses is not unusual, although you usually see the reverse. That is, you usually see darker spots, such as Bend d'Or spots on a chestnut. However, I have known of other dark horses that have had sorrel spots, and mahogany bays that have had lighter sorrel spots. Little is known about them - it is thought to be just a coat developmental variation. However, since it seems to run in some families, there may be a genetic component.

In the case of your black filly, because the sorrel spot has been diluted, it makes it more apparent than would be noticed on most horses. "