An atmospheric layer of particles (such as from volcanic activity or forest fires) between an observer and the moon scatters red light to leave direct light from the moon a blue colour. This is the origin of the expression "once in a blue moon" because this is a very rare occurrence.
In September 1950 smoke from a Canadian forest fire drifted across the Atlantic, and Europeans were able to see a blue moon.
In my book "Seeing The Light" (ISBN 0-9735316-1-4) I speculate on the possibility that "once in a blue moon" might be a daily event on the planet Mars because of the dust in the atmosphere making the sky pink. The APOD picture for August 4, 1997 shows a "salmon" coloured sunset on Mars, which might be explained as resulting from Rayleigh scattering removing most of the blue while Mie scattering from dust particles removes some of the pure red leaving a significant portion of green wavelengths to balance the remaining red to the more neutral "salmon". Does anyone know if the size of dust particles in the atmosphere of Mars is appropriate to produce this "blue moon" phenomenon?
once in a blue moon
This looks much more "blue", indeed. EDIT: please continue in this earlier thread - locked.