Haven't posted for a while but had to when I saw this thread. There are at least two different alleles responsible for blue rats. One is d/d which is a simple recessive and modifies or rather dilutes(hence the d) black to a russian blue. The other is what most breeders refer to as american or slate blue which is also a simple recessive. I would imagine it would depend on the breeder and how inbred a particular line is, therefore blue rats would not inherantly have any more health issues than say, a siamese. That is just silly.