Invisalign retainer has a dark mark on it. Should I be concerned?

Trays
and retainers from Invisalign do have writing on them for purposes of identification
in black ink. It is not toxic to the
body at all. You should show it to your dentist if you are concerned

Thank you for posting your question.  Without seeing what exactly your referring to it is difficult to give you advice.  Having said that, there is a possibility there is a manufacturing defect in the invisalign retainer.  Ask your dentist what it could be if you are not sure.  Invisalign is pretty good about sending you a new set if it does happen to be a manufacturing defect.  Hope that helps.  Good luck!

Are you looking at the numbering system that identifies the case the retainer is for?  Probably.  As to a chemical taste, this is a different material than the aligners but I have never heard of a complaint (my wife wears one).  Scrub it with a toothbrush and dish soap in lukewarm water (not toothpaste -- has grit in it) and see how it goes.