If it's a good blade, is the towel you use to dry it clean? Might there be something on the towel, either from use, or from detergent, or fabric softener, that would be left on the blade after drying and therefore under the coating of oil you use for protection?

It's a pitty that a mirror blade would end up looking like that when you appear to be taking every precaution to preserve it.

Best of Luck,
Jim