A bit taken from a talk given by GBR at a seminar [You must be registered and logged in to see this image.]
The Cobalt mutation is strikingly black and luminous but at first glance it might be confused with a classic melanin bronze. The cobalt hypermelanism mutation disperses a melanin veil effect over the lipochrome that occupies the outer edges of both sides of the feather giving the bird a striking darker black color. The phaeomelanin (brown) which is commonly express in the feather is no longer visible resulting is a clear, dark color and smooth ground color with maximum dark eumelanin. Geoff remarked that an onyx cobalt is the closest thing today to a black canary!
The cobalt should be luminous, especially evident in the intensive feathering and not dull. Selective breeding of these birds has resulted in a loss of all brown producing genes because of the optical blue factor or perhaps the Azul factor which he is currently studying. The azul factor is like a predictable optical blue, shiny and beautiful and never dull.
The feet of the cobalt should be dark in the bronze cobalt just like in a classic bronze. Classic bronze typically show lighter feet the second year but at this time some cobalt may have lighter feet, a fault, their first year.
Cobalt is a recessive mutation and follows the standard laws of Mendelian inheritance for a recessive mutation. For a bird to be phenotypically (visibly) cobalt it must inherit the cobalt gene from both parents as it only shows when the bird is homozygous or in other words when it has two cobalt genes, one inherited from each parent. This inheritance pattern is seen in a number of colorbreds such as opal or recessive white etc.[You must be registered and logged in to see this image.]
the distinction between the intensive classic bronze and the intensive bronze cobalt which is easily seen by simply turning the birds over and examining the vent area. The classic bronze shown on the bottom, has considerable red lipochrome color on the underside while the Cobalt picture on the top is black clear to the vent due the super imposing of melanin pigment over the lipochrome on the feather edges!