Amy:

This sounds similar to our blue-haze artifact that is seen from time to time.  I am not certain that it is caused by the same thing every time, or that we have actually found the answer, but this is our experience.  Every time it has been seen since I have been supervisor, it has appeared to be due to water in the xylenes and or paraffins in the processor.  Or it has appeared to be due to water in the last xylenes when staining.  These are two very different problems, but the pathologist's description of them here is similar.  It sounds like you have ruled out a staining problem which is the easiest to fix.  

If you don't find water in your xylene and/or paraffin, there are some other options.  Previous supervisors had received numerous complaints about this blue-haze.  One thought that it had something to do with heat.  The heat was taken off all solution stations on our processor (which is still the case).  Another time the blue-haze was blamed on the heat on the embedding station.  The theory was that the embedders were leaving the tissue directly on the hot plate for an extended length of time.

I have my doubts about the heat, but we don't have as many blue-haze complaints now as we did then.  So we still have no heat on our processing solutions, and the embedding techs try to embed biopsies as quickly as possible after removing them from the paper to lessen the contact with the heat plate.  I guess it is just part of the voodoo that we do to avoid the dreaded blue-haze :)

Good luck and I will be very curious to know what it is if you find the cause.  

Ross Stapf
Histology Supervisor
Washington Adventist Hospital
Takoma Park MD