Is Condensation on Windows a bad sign?

Condensation on double-glazed windows in cold climates is fairly common unfortunately. Despite being common it can be a problem, particularly in extreme cases, which yours seems to be. First I would point you to a page we have that should give you all the info you need –

Condensation on windows – what causes it and how to stop it

And to address your case specifically, I’m not surprised that it is worse in the bathroom, that’s where the most moisture is. But running all the way down the wall to create a puddle on the floor is quite a lot. What you really need to do is address the high level of humidity that I’m quite certain you have in your home, because you are already seeing the signs of damage in the form of black mold on your window frames.

In order to be able to fix your problem, I would first recommend you get a hyrometer, which will measure the relative humidity in your home. They aren’t expensive (probably in the $20 range) and it will be great to have in the future to keep an eye on the humidity levels.  

As for the source of humidity – here are a few more questions – do you have basement and is it humid? Do you have and use a stove hood, and does it vent out? Do have you have (and use) a bathroom fan? And how old is the house - do you know the year it was built?