Merles are obviously not blind, Jasper see's just fine. There is a lot of controversy about the Merle color in chihuahuas because its supposedly not a "natural" color combination and therefore must have been bred in from another breed. That is not here nor there for me. I have always LOVED the Merle coats and when I realized I could have my favorite dog in my favorite coat I set out to have one. And, I lucked out totally, he's gorgeous and has a GREAT personality.
I do understand that there is a danger in breeding Merles, especially if you breed 2 merle carriers together, that causes defects including blindness or eye issues. The same can be said of other genetic color combinations in other breeds/animals (IE: the "lethel white" in horses). To breed merles responsibly, one must be sure to not breed 2 carriers together. As with many other traits, the merle gene might not be apparent in the parent coat but the parent can still throw the gene.
Again, I'm not a breeder and I am not concerned with all the shoulds and should nots, I think there will always be contrasting opinions on a variety of aspects of any breeding issue. I do not plan to breed, I am having nuetered, I checked his parentage and to the best of my understanding he does not the dual gene. I am being responsible.