"A very rare phenomenon known as a 'blue sun.' These have been observed from time to time around the world. The most recent widely accepted documentation of a blue sun occurred in September 1950 in Edinburgh, Scotland. A discussion of that event appears in Craig Bohren's book, 'Clouds in a Glass of Beer,' on page 91."
The main cause of 'Blue Sun' is thought to be smoke in the atmosphere. From time to time smoke from farmers burning brush between the camera location and the pyramids' plateau rises to camera level and is apparent on many of our images. Link