Jay, if you have mites, you've got to seriously consider stopping breeding and starting over from scratch.
And yes, the blues most people refer to as being weak or ill lines are the English blues. Russian blues seem to have no problems, at least from the lines I have been working with.
And no, it's not JUST a color, there is more to the colors in rodents and mammals than just a paint job. Take a look at high white animals, horses, dogs, cats. Often they are deaf (along with more serious diseases), this is due to several factors, one that contributes highly to delay in the migration of the cells that create pigment to the entire body.
Secondly, blame the rodent fancy/pet trade for the English blue problems. Eng. Blues were severely inbred to the point where hemorrhaging and other fatal diseases were rampant. We can still see much of this today, luckily, the Eng blue gene is recessive, and can be worked away from by careful out breeding. I personally have only seen trouble with the English blue color. Russian blues, not a problem! (And yes, they are very different colors, created by two very different genes)
Claiming that color has no other effect than to change the fur color is not correct. I'll step off my soap box now.