Q: We received some red oak lumber and a few pieces are blue in the sapwood. Is this mold fungal stain?

A: Although there is a blue stain fungi that gets into the sapwood of oak, it is not truly a mold fungi. This blue stain fungus does make the sapwood look bluish. I suspect what you have is the enzymatic oxidation gray stain that occurs during warm weather when drying is slow or nonexistent. In oak, this gray stain is quite common, compared to fungal stains. For more detail, see  forest.wisc.edu/extension/Publications/64.pdf .