I've got american blues and dumbos, haven;t yet produced a blue dumbo but produced plenty of blues and dumbos and my blues seem just as healthy as any other rat. I've outcrossed the blue line a couple times, but producing blues doesn't take THAT much inbreeding, just one generation really.

I don't think there is anything inherent in blues that makes them less healthy. Some blue lines have probably been inbred excessively which might account for it.