In case of interference,we know,energy is neither destroyed,nor created; but only redistributed.But in the case of an extremely thin film,due to a reflection and hence a phase difference of pi, the film always appears dark due to destructive interference. So,where does the energy go?
The logical answer is : into an increase in the motion of the atoms on which individual photons scattered off the thin film, i.e. heat.
This is a fascinating similar phenomenon with monochromatic laser light showing destructive interference. It is instructive to look, as it shows the quantum mechanical dependence of light.