I have had the blood allergy testing done on me and it was spot on at finding the stuff that doesn't give an immediate reaction, but the stuff which caues long term issues (headaches, not feeling right, etc.).
I have found the blood testing by VARL and Spectrum to be the only way to get my dogs straightened out, being surprised by results, and seeing improvement when they were aaddressed. These folks that downplay the results of these tests clearly have never really used them properly. I think the spectrum is most comprehensive, it was my choice for the last two dogs I tested.
You still have to challenge, since sensitivities are different than allergies, BUT, the information of where to start and what to avoid is really helpful.