Size? Shape? Composition?
I can't think of anything that would come up blue. Otherwise, there are several native fish species that actually have these rock type things form in the stomach, as a result of the foods they eat (chiton, bones--indigestable stuff). These stones are occassionally vomited up. They have a specific name, I can not think of it right now. It may be something similar, if you can identify the source of the color in something you feed regularly that would be calcareous or chitonous.
otoliths? No, that has to do with the inner ear...