Maybe I can help to explain a bit more. The polarizer is always blocking light but it is most effective with scattered light – and its not that simple. When light is scattered off a reflecting medium (whether that is water, glass or particles in the atmosphere) it does so with what some scientists call a preferred direction. If you want to eliminate that scattered light then you need to rotate the polarizer so that it aligns with that preferred direction (and that is why polarizers come with the ability for circular adjustment independent of the screw thread). Light which comes to you with a different preferred direction, or no preferred direction at all (i.e. ‘normal’ light) will still get through (and this is why everything doesn’t go black – there’s always either a bit of ‘normal’ light or light with different preferred directions around).

So what is that preferred direction? It depends on the direction that the light is coming from and then scattered to. So for the landscape photographer, say, the source of light is usually the Sun and you with your camera is the direction that the light is scattered to (with the scattering medium in the middle). Now the preferred direction will be in a direction which is 90 degrees from those two lines. For instance, if you are framing a picture of a lake then the light comes from above and is scattered to you from the lake surface. The preferred direction of that scattered light is parallel to the lake surface, and by rotating the polarizer to the correct angle you can cancel out that scattered light, which is usually the glare that you see (or specularity as some refer to it) e.g. from ripples on the lake surface. Look up at the sky and rotate the polarizer and you’ll see that different patches of the sky darken as you do. That’s because for the sky, those preferred directions rotate about the Sun. Shoot in the early morning or late evening (and looking to the North or South) then most of the sky will darken when you angle the polarizer appropriately as the sunlight is coming from predominantly one direction.

Also, the polarizer is always blocking off light, but when that light is direct (i.e. it hasn’t been scattered off a surface) then there isn’t a preferred direction and it only serves to cut the light intensity down. I use this to good effect sometimes when I want to lengthen my exposure time.

Gosh! I’m really sorry, that was long winded! I hope it was informative and not boring though….!

Thanks ThoSo

While this is probably true (and the effect sure is), something is missing. Except for skylight and actual lights all light is reflected so the filter can’t block all “direct wavelengths”. If that was true everything except the sky would be pitch black.

Am I wrong?

It is all about the polarization of the light, meaning the direction of the light waves. Without any filter, in principle all directions are can be found at all wavelengths (red to blue). Now, since light (especially blueish light) is “reflected” or better scattered at the sky, especially at lower angles of the sun we see a polarization effect on this light. A polarization filter is able to either let exactly one polarization direction through. So it can be used to suppress light originating from the scattering. Similar effect take place when light is reflected from e.g. glassy surfaces, hence a polarization filter can be used to remove the reflected light in similar conditions (depending on the angle of light and surface). Btw., the same principle is used in connection with LCD displays.
Cheers, Thoso