jrdoran ... hi, and welcome to the forum. Hope you will find answers to your questions about ALS.
I've never heard of any blood tests that would detect the presence of ALS.
As to abnormalities that point away from ALS ... that depends on the abnormality. For instance, if someone has Myasthenic Gravis, this would often show up with the proper kind of blood test, and would rule out ALS. However, MG would not show up in a run-of-the-mill blood test. ... it takes very specific kinds of blood work to detect.
Aside from ruling out "mimic diseases," however, there is no reason that one could not have both ALS and another condition that would show up in a standard blood test, such as anemia.