Do you know what kind of heritage breeds make Production Black hybrids?
They don't say it on their website, but they got them by crossing white leghorns and brown leghorns. I know this because that's the information they used to publish in their catalog.
White Leghorns are dominant white, but most strains carry the barring gene hidden underneath that white. So if you cross White Leghorns with a non-barred breed, and then mate the resulting offspring together, chances are strong you will end up with some barred birds. That's how Ideal ended up with barred leghorns, which they call Production Blacks.