Blue stain (aka sap stain) in wood is caused by a fungus that under certain conditions will spread into wood once it is removed from the stump. The fungus eats nutrients found primarily in the sap, so it usually only occurs in the sapwood and not the heartwood. Once present, its effect can be somewhat mitigated, but it cannot be removed completely. It does not occur in live trees because live sapwood does not contain enough oxygen to sustain the fungus. It is common in pine and maple but also occurs in many other woods such as anigre and cedar. The color is usually blue but may also be gray and or even dark gray verging on black. The color is the fungus itself rather than a chemically induced change in the color of the sapwood. There are extensive articles on the internet discussing it and how to avoid it and even how to reduce its effect (but once it's there, you can't get rid of it totally --- you need to avoid it during drying). It does not normally start to affect logs for many days, sometimes weeks or more, and of course, sometimes not at all, but it CAN start immediately after cutting and have a pronounced affect within just a few days. One of my correspondants who uses a lot of aromatic red cedar tells me that it commonly occurs in that species within days of the tree being cut.

The fungus can reach a tree as airborne particles or it can be deposited on them by flying insects, such as the pine beetle, which get it from the air or from another tree.

Ebay does not allow links to pages off of ebay so I can't put any links here to my own site, which has pictures of woods with blue stain, or to Internet pages that have a more full discussion of blue stain, but you can find it all pretty easily. If you search for SOME WOOD TERMS AND EXAMPLES you'll find my site with pictures.