Ayam cemani are always black, that is what the name translates to (black chicken). I have had recessive white chicks pop up in my line of AC, but I always cull them and stopped using that rooster to minimize the chance of the recessive white popping up again. Anyone trying to pass off any non-black bird as a "pure" Ayam cemani should be avoided, there are enough poor breeders of this breed without encouraging weird mutations and crossbreeds under that name. It is not easy to produce high quality Ayam cemanis, but it is easy to make crossbreeds or select the lower quality birds and pass them off as "special". The fastest way to ruin your reputation as a cemani breeder is to sell off type birds as pure.
If you want to turn a single white sport from a line of AC into a whole line, it would be trivial to do so, but please don't call them "White Ayam cemani", make up some other name that does not have "cemani" in it. If you told someone you had a "White Black Chicken" what would they think? Maybe a barred rock?
If you cross an Ayam cemani and a white Silkie, you would retain much of the black tissue, and the resulting black birds would carry 2 recessive genes, white and silkied. By crossing those F1's, you would get a whole range of birds, some white non-silkied and some black silkied. But neither would ever be a real cemani.