Well, there is another possible explanation.  Sometimes the surface glaze can have very small cracks (not always visible) in it and moisture can get underneath the glaze.  Sort of like crazing, but not in the typical crazing pattern.  The cracks might come from wear, perhaps a default in the glaze itself, age, or any number of other causes, of course including a dishwasher.  (Perhaps from temperature variations or the chemicals in the  washing powder?)

Before eating from them, I would want to make sure that it is a surface thing and not an issue with the underlying pottery itself.  Either way, you got yourself a good deal, so it's not like you are really out anything.

Perhaps try soaking one to see if the grey streak increases in width?  If it does, you will know that the glaze is no longer preventing contamination of the porous clay underneath.

Worst case scnario, you have some great matching saucers for potted plants!  Maybe some pieces for interesting matching yarden art?