Just because a Hereford animal has some black hairs doesn't mean his pedigree nor purity should be in question. I've seen hundreds of animals over the years with black hairs in tail or an occasional black patch on the body. The old timers always told me that black hair on the tail is a sign of fertility in bulls.
Dr Beevers comments from another site. "In regard to the occasional Hereford animal (other red animals also) that have a patch of black hair somewhere on their body, these are most likely the result of a phenomenon called "reversion." Because the red allele is non-functional, occasionally some individual cells have spontaneous mutations that revert the red allele back to the black. Every cell that is derived from the original mutant will produce black pigment, thus the patch. The patch is localized because as the cells divide they all stay localized to the same area."
"For those animals that have black on the tail, etc... it is most likely due to the selection of modifying genes affecting the base color. I think a lot of this comes from some fairly intense selection on darker color in some lines of cattle. It used to be somewhat restricted to dark polled lines, but you can occassionally see it in more horned lines today."
