Theoretically, you could have a high-percentage Hereford that is black, but it's highly unlikely. If you breed a black animal (BB) to a red animal (rr), you will get a Br animal. Since black is always dominant, you will get a black calf with a dominant black (B) gene. However, this calf will also be carrying a recessive (r) gene. This calf's genetics will be Br. When this calf is mated to another red (Hereford) animal, it will have a 50-50 chance of having another black calf (again Br). If the resulting calf is again black (Br), it will carry the Br gene just like the preceding generation. Another generation will have a 50 - 50 chance, and so forth for each succeeding generation. If at any point in these generations you get a red calf (rr), the black gene is totally gone, and you will have to start over. But---as long as you continue to get black (Br) calves, you have a 50 - 50 chance of continuing to get black calves.
If you mate two Br animals, mathematically you will get 25% BB (homozygous black which will ALWAYS give you black) calves, 50% (black with red gene) Br calves, and 25% (red with no black gene) rr animals.
It also works the same way with polled vs horned cattle, with polled being dominant.
Clear as mud??? Hope I have explained this so everyone can understand it.