Are your birds from a hatchery? Hatchery birds may have interesting recessive genetics that sometimes come out when you breed them. Not to mention, sometimes a white bird will carry recessive black genetics that may end up trumping a red bird's genetics... rarely.
The one thing I can think that makes more sense is that a white rock roo carries recessive black and your California white hen carries recessive black (because of the barred California grey in her heritage used to make the cross), and when crossed they produce 25% black chicks with recessive black genetics. But in this instance, you should still have some white chicks.
You also said the White Rock roos and the California White hen were kept separate, so I don't know.
Is EVERY single chick black? This could be coming from your hatchery quality production red roo with odd genetics. Any chicks with other colors?