Just like with the ayam cemani you will always have some to display mulberry in the comb as they age. Not every bird is expected to hatch being perfect. Just like with any breed a percentage are going to be best quality (dark-dark) some are going to be regular, normal quality (dark maybe a little mulberry.)
According to which standards?
The origin country standards are quite specific to them being all black, and with true stock its very easy to see all black.
http://en.wikipedia.org/wiki/Ayam_Cemani (the pictures here are of not so great US birds, but it doesn't say mulberry does it)
http://www.cemanifarms.com/2013/10/ayam-cemani.html
http://www.longtail-fowl.com/cemani.html
Notice on this page that the greenfire farms pictures are obviously different because of the mulberry coloring... Not real Ayam Camani...
http://www.feathersite.com/Poultry/CGA/Cemani/BRKCemani.html
A good description first hand of the inbreeding and mixing of Ayam Cemani and Kedu
http://home.planet.nl/~stev7596/avisitto.htm
Again the locals saying that Ayam Kedu is often sold as a Camani and used when farmers need a quick cheap way to make a buck.
http://home.planet.nl/~stev7596/gatra.htm
An explination of the Ayam Kedu breed that includes the explination for the two types: White Kedu and Cemani Kedu
So its easy to get confused, but its a Cemani Kedu, not an Ayam Cemani when it shows black traits, but not true black ie. Mulberry colorings.
http://www.temanggungkab.go.id/potensi.php?mnid=84
If it's not
all black, then its an Ayam Kedu. People will try to sell the all black Kedus as Ayam Cemanis, when they are Ayam Cemani Kedu.
Those black birds making mulberry combs are simply not real Cemanis, they are mixed or close to, but not real.
Take a look at any picture of a "Cemani" from a US site and compare them to any international source and you will quickly see that the US breeders have a different stock because of all the Mulberry coloring in the combs. These are not true Cemanis, that level of coloring can be reached in a first generation mix.
I did a lot of documented fibro experiments, and its quite simple. Any Red = Mixed fibro genetics
The ONLY time a true fibro bird has any red is if they get a VERY rare albino gene that results in white plumage with all other fibro qualities remaining. Some of them will have a light mulberry that is of the same quality throughout the flesh, not in blotches, a consistent mulberry black tone.
Solid Black can only be had by a true fibro to fibro pairing, or luck of the draw with fibro to weak or other pairings.
Why is this so hard to understand?
...Oh right, because an untrue Cemani can still fetch a really high price from an unknowing buyer.