Only two ways this can work:
1) Cow is not yellow. She is essentially a black cow with color dilution or/and color inhibition gene altering expression of the black gene.
That, or 2) the sire is not the Hereford bull, but, rather, a black-hided 'visitor'.
Had a couple like her.. half Angus, out of a little yellow cow that was daughter of a little yellow whiteface lineback Charolais/Polled Hereford-cross cow and a red/whitefaced dilution-gene-carrier SimAngus bull. Bred to Angus bulls, they always dropped a white calf, but bred to Shorthorn, calves came out black. Other color possibilities could have come out, but that's how the genes happened to shake out on those 6-8 matings.
These girls were all but white, with black nose & hooves, and at times their hair coat took on a dingy yellow discoloration..but they were not 'yellow'.