It's a contraction of "a nod is as good as a wink to a blind horse", meaning that if the person you're talking to has no idea what you're going on about, then you can say/do whatever you want and they still won't get it.

As such, it's actually quite inappropriate here, and a more effective phrase would be "a blatant come-on's as good as a subtle hint".

Ahem. I shall now try to stop sounding like such a grammar ponce. Sorry.