You don't need to do blood tests to "see" the effects of being Heterozygous for the Sickle cell gene. The rules apply for things we can see with the naked eye and those we can not. The genetics we commonly look at in the hobby are externally visual ones, eye color, skin pattern, pigmentation, etc.. That does not mean that all the traits we are not looking for don't exist. We just don't care about them unless they affect the visual morph of the animal.
Anyone who breeds corns, or other snakes, long enough can tell you that they can pick out animals Het for certain genes.
Terri