Quote Originally Posted by AgX View Post
It is the red light that passes more through haze than blue light.

That means if on a very hazy day you want to "cover" something in that haze, or with other words to emphasize on that haze, a blue filter would do so.
Notwithstanding whether the light source is within or outside the haze.

In colour photography the resulting blue effect could be filtered out in a later stage.

Sorry, I beg to differ.

Water vapor and most particulates and pollutants absorb or scatter the longer wavelengths and pass the shorter (blue end of the spectrum) (this is what makes the sky blue, not red...).

Anyone who has been more than a few meters underwater (snorkeling or scuba diving) knows that everything turns blue; the longer wavelengths (red, yellow, green) get filtered out because they are absorbed; the blue passes through. The same thing happens with water vapor in the air. Cloudy days are markedly more blue than sunlit days for precisely this reason; the longer wavelengths are filtered out by the clouds.

UV or yellow filters are sometimes called "haze filters." They "eliminate" the haze by darkening it, i.e., by blocking the predominantly blue (and near UV) shorter wavelengths making the haze darker and less noticeable in the print. Using a blue filter passes those shorter wavelengths and makes the haze lighter, thereby "emphasizing" it.

A clear UV or haze filter is what is generally used in color photography to "eliminate" or "reduce" haze, which it does by blocking the near UV and a bit of the darkest blue. These are the wavelengths that are predominant in haze.