If you want to block out all light, how should you arrange two polarizing filters?

Indeed, this is one of the odd results in physics.  A system of two polarizing filters arranged as shown below trasmits no light.

However, if a third filter is added, then some light will passthrough.

This is because a polarizing filter changes the polarity of the light passing through the filter.   It is not knocking out "undesirable" light, so much as modifying the polarity according to the orientation of the filter. 

Since the 0 degree and 90 degree filters are not compatible in polarity, no light passes the filters.

However, since the 0 degree and 45 degree filters are compatible, as are the 45 and 90 degree filters, some light passes through the 3 filter system.