The Skagit Valley Herald published this photo of a Melanistic ("Blue Phase") Snow Goose in the Skagit flock
Very nice catch.
On a technical note the "blue phase" is the same as the "white phase" except the feathers are colored with eumelanin.
This is controlled by a single gene, MC1R, in a huge range of organisms e.g. mammals including humans, birds, fish, amphibians and reptiles. A nice example of evolution sticking to one method when it works.
MC1R been shown to control melanism (i.e. dark phases) in the bananaquit, the snow goose and the arctic skua in this paper published by the Royal Society from the Evolutionary Genetics Group at the University of Cambridge:
Lesser snow geese show an approximate east–west cline in morph frequency in their breeding distribution across the nearctic into the eastern tip of Russia, with blue morph birds commonest in the east. Extensive studies of fitness components in a colony at La Pérouse bay, Canada, failed to find any adaptive advantages related to plumage morphs (Cooke et al. 1995). However, geese show strong mating preferences based on parental colour, which leads to positive assortative mating (Cooke et al. 1976).
I presume this is a single mutant bird from Wrangel Island rather than one of the other populations where the "blue phase" is more common.
The melanistic gene should be dominant but the birds breed associatively so this bird might have a hard time getting a mate. But if he does find a mate his offspring will be "blue phase". So that's a good reason to check again over the next few years (the geese don't breed until they're 4 or 5 years old) to see if dark phase numbers have increased.