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Black is just the lack of any other genes. Blue is its own gene, when a bird is not carrying it (or anything else that would allow it to be seen phenotypically), a bird is black.
There are both dominant and recessive whites.
First, the genetics for silkies are the same as for all chickens. With secondary patterns (patterns on individual feathers), the
appearance can differ due to the lack of barbicels on the feathers; the patterns are fuzzy and indistinct.
Chickens carry only two pigments for their plumage colouring: black (eumelanin) and red (pheomelanin). Genes can dilute or enhance these pigments, move one or both pigments to different parts of the body (primary patterns) or different parts of a feather (secondary patterns). White feathers or portions of feathers lack all pigment.
Blue is an incompletely dilution gene; it dilutes all eumelain carried by the bird. One dose (Bl/bl+) dilutes the bird to a slaty grey; two doses (Bl/Bl) dilutes the bird to a splotchy grey/black on a pale to slaty grey background. The first is called blue; the second splash. Shade/tint of blue and splash vary considerably. Blue has little to no affect on pheomelanin.
The "neutral" chicken is wildtype, the genes present in the red jungle fowl. These genes are indicated by a "+" This phenotype in a chicken is black breasted red, also called gold duckwing. Red jungle fowl from feathersite:
There are actually several different "blacks." Which type is determined by the E gene. The most dominant of these alleles is E, extended black. Ideally it causes a completely black chicken, but even E needs melanizers (additional genes that add black) to create a completely black bird. Next in dominance is E^R, birchen. E^R creates a bird with pheomelanin on the head, hackles, shoulder, saddle and breast feathers; birchen-based birds do not have pheomelanin on their secondaries (wing triangle). Here are examples from feathersite:
add the blue and gold genes and you will get:
(hobby name lemon blue)
Here is a female with the birchen pattern; she also carries the gold gene:
(hobby name brown red)
Next in dominance is W^Wh, wheaten. This gene has a variable dominance: when melanizers are added, it becomes the most recessive E-allele. Pheomelanin is present on the head, hackles, shoulder, secondaries (wing triangle), and saddle feathers on males; on females pheomelanin is distributed over most of the bird, with eumelanin limited to the tail and a bit on the wings. These birds from feathersite are blue wheatens, meaning that the blue gene has been added, diluting black pigment:
Next is e+, wildtype or duckwing. Pheomelanin is present on head, hackles, shoulder, secondaries & saddle for the male; on the head, hackle & breast of the female. From feathersite:
And finally, e^b, called either brown or partridge. Male pattern is similar to wheaten and duckwing; however the hackles have black shafts; female pattern is similar to duckwing, but the breast is not salmon. e^b works extraordinarily well with secondary patterns, unlike E which only works with mottling and barring patterns. I could not find any photos of e^b birds without added pattern genes.
Most silkies are e^b based; however as new varieties are introduced and worked on, I suspect that other E-alleles are also being added.