Several months after Pendaran died, Blue started looking a bit off.  At first, there was nothing I could really put my finger on and tests didn't turn up anything, but eventually she quit eating and had to be hospitalized for a while.  I force fed her for some time before she started eating on her own again.  Tests continued to show nothing until one day, an in-house Feline Leukemia test came up positive.  There was part of our answer - or so we thought.
Blue did very well on steroids for a long time.   Subsequent Feline Leukemia tests were all negative.  One day though, I came home from classes to find her looking bloated in the abdomen and in some discomfort.   I watched her for 20 minutes but could tell she was rapidly getting worse, so rushed her to the animal hospital.  The vet did an exploratory surgery to find out what was going on, but was unsuccessful in coming up with a diagnosis.  Blue died without regaining consciousness after the surgery.

All those years I'd wondered exactly what had been wrong with her, then Roo got sick in the summer of 1999.  His symptoms were very similar to hers (the original symptoms, and not the ones just prior to her death).  We are fairly confident that Roo had intestinal lymphoma, and I now believe Blue Moon had the same thing.  The lone positive Feline Leukemia test may have been a false positive, or the cancer may have been secondary to the Feline Leukemia.