Whites are the original colour for silkies, so why would breeding to blacks produce better birds? makes no sense. Sometimes the other way round, but blacks have been around long enough that many have excellent type.
White X black will not improve white--it will just bring out their hidden genes. Breeding a lower quality bird to a higher quality bird will improve the chicks as compared with the lower quality parent.
Well, there could be blues, but I see no reason to expect them.
Cull means remove from breeding stock and nothing more. People use different methods--personally I only sell or give away birds that do not fit my needs.