While some double merles are blind, deaf or both, most are not. It can take up to five years for their hearing or vision to come in fully. They can be both a detriment to the breed or an asset. I have been a breeder for over 3o years and only bred blue to blue once. When doing this you should be aware of the genetics behind the breeding so you can prepare for what you would do with such handicapped puppies. However, the benefit to the breed is the beautiful clear blue silvery blue coats they produce. When you see dogs whoose coat id getting more pewter gray, they should be bred to a double merle to produce that coat color so desired in the breed. Some are so dark gray as to look more like charcoal than blue. Yes there isa risk for defective puppies, but double merles have also produced some fantastic dogs that are out there.
The ones that I object to most are people selling sable merles who do not know the genetics behind the dogs and sell them to persons who breed them without understanding the genetics and then have problems with their puppies. Every person buying a sable merle should either NEED to know the genetics before breeding or they should be neutered. When I see people advertising RARE color headed whites or mismarked whites or double merles as RARE I want to choke them, they are misleadig and hurting the breed. In the same way the breed should always identify sable merles and advise the buyer of the effects that are possible in breeding these dogs.