Don't do this:
First of all: Traditionally the way to communicate a sneaky surreptitious intent is winking, not blinking. (Blinking is involuntary and therefore hard to use as a code.) Second: The reason to communicate by winking, or blinking if you must, is to avoid saying out loud the thing that you're not supposed to say. Actually saying "blink blink nod nod" defeats the entire purpose. Third: All of this is extra true over e-mail. It lasts forever. They can search it.