I woke up this morning with a coating on my tongue that was a blue-green color.  I thought over what I had eaten yesterday, but nothing out of the ordinary and nothing in those color families.  I am a 66 year old female who is currently undergoing radiation to the retinas.  I completed six rounds of high-dose methotrexate this summer for Primary Intraocular Lymphoma (so rare almost no one in the world has had it).  I have had no side-effects (except being quite tired) from any of my treatments, which has amazed everyone, including my doctors.  I also did not lose my hair from the chemo.  I hope this blue-green tongue doesnt indicate anything serious.
Thanks for your query, I have gone through your query. The bluish green discoloration could be because of the chromogenic bacterial infection. It can be secondary to radiation therapy where the salivary secretion will be reduced. Consult an oral physician and get yourself examined completely and take a course of antibiotics and follow thorough tongue brushing. I hope my answer will help you. Take care.