The blue gene is a recessive colour gene. Both parents of a dog must carry this gene for a pup to be blue. At birth, dogs who are blue will be 'grey' or 'silver' in colouration, not the usual black. The blue gene is a Dilute gene, meaning it dilutes all the black pigment of a dog. The blue colouration can very from a very light 'powder' blue to a very dark, almost indistinguishable 'steel' blue. Blue dogs will also have very light eyes, often being very blue at a young age, and as the dog grows, going through varying shades of green and yellow, often ending up yellow or a light shade of brown.
taken from:
http://www.4gsd.net/colours.html
Hope this explains it