From the metzer farms website:
http://www.metzerfarms.com/BlueSwedishDucks.cfm?Breed=Blue Swedish&BirdType=Duck&ID=BS
http://www.metzerfarms.com/BlueSwedishDucks.cfm?Breed=Blue Swedish&BirdType=Duck&ID=BS
So that we can produce 100% blue colored Blue Swedish, we have two breeder flocks. In one flock we have Black Swedish males and Silver females. In the other flock we have Silver males and Black Swedish females. All the progeny from these crosses will be the correct blue color. In June when we need breeders for next year, we switch the males so Black are on Black and Silver on Silver. We collect these eggs for about three weeks and hatch them for breeders. Then we switch the males and go back to producing the correctly colored Blue Swedish for the rest of the season. This is the reason we do not have Blue Swedish or Blue Runners available in July of each year.