I contacted Dr. Jacobson, who has studied IBD at the University of Florida. He basically said I should be concerned, and have two tests done. The blood test seems inaccurate -- he said more often then not "he sees false negatives more than false positives". That's an unnerving prospect.
This is correct, which is why they recommend two blood tests at least 90 days apart - the likelihood of two false negative results in a critter with IBD is fairly low.