It seems like all the eggs you hatched were from the darker hens or the roosters even darker and more dominant genes made those chicks black, however it seems like the Orpington hen hatched two of her own eggs
Black or blue color genetics are much more dominant, that is why if you have a pair of white and black silkies 50% or higher will be black. However this can depend on the breed or whether the hen was black or the rooster. Can you give pictures?