Originally Posted by Wheatfield 
The fix it in post have got it wrong in my experience. Sensors are "daylight balanced" in their native colour sensitivity range. You move outside that range, and white balance will pick up the slack to a point, in much the same way that with film, the printer could give you a reasonably acceptable print in some situations where you moved outside the film's spectrum of acceptability.
Move to far away from what the camera can accommodate, and you will likely never be able to get an acceptable white balance.
True. But the problem with colored filters is that most of the situations we are talking about here involve low light levels as it is. A blue filter means it's going to be that much hard to get enough light at all. Of course, one could question whether bad light is better than no light. But I do find that if nothing else, the red channel on its own often makes a fine B&W conversion.