The whole basis of blues being sickly is from the idea that pet shops around the country saw that the english/american blues were a huge hit, and from there they were mass produced and not selectivly bred. The major inbreeding and line breeding brought about poor immune systems, inheritable conditions and just a poorly bred and adjusted rat.
Many blues bought from the pet store are said to be sickly, mean and die young.
I think on the good side, the rats being bred for feeders aren't (usually) being bred for color, and whatever color or pattern we get is purely random. That outcrossing for several generations, and the "survival of the fittest" idea could very well have given you a line of healthy american blues.
Outcrossing your own blues is different than intentionally picking out a blue rat from a pet store situation and expecting it to breed and be healthy.