Welcome to the Shroomery Message Board! You are experiencing a small sample of what the site has to offer. Please login or register to post messages and view our exclusive members-only content. You'll gain access to additional forums, file attachments, board customizations, encrypted private messages, and much more!
This doesn't sound like a contaminant. Probably it is just "bruising" which indicates the presence of psilocin. Not unusual. This may or may not be an abort:
It is often difficult for beginners to identify an aborted mushroom before it begins to decompose. Early warning signs include a halt in growth of the mushroom, and a greenish tinge around the dark colored tip of the primordia that will eventually become the cap of the mushroom. -- http://www.mycotopia.net/teks/scgg.html