Norm:
Most likely your small sticker didn't allow enough air flow through your stack. And the moist air just sat in there and that's what the blue stain needs to help it grow.
Increasing the size of the stickers should help. There are safe chemicals that you can put on timbers to stop mold and fungus.
If you go to the
www.woodweb.com and do a search of their knowledge base there should be lots of information about fungus and blue stains.
Good luck,
Jim