1608, July: Blood Rain Discovered in Aix
Towards the beginning of July, 1608, the residents of Aix, France, were horrified to see stains that looked like large drops of blood had fallen in their village and in small towns and villages for several miles around, staining all the walls of of the buildings, and even the walls of a cemetery near the city. It was thought by many that this rain of blood was the work of sorcerers, or of the Devil himself.
When is a Rain not a Rain?
The solution to the mystery was suggested by a man named de Peiresc who made a chance discovery at the right time. Having collected a crysalis in a box a bit before the 'blood rain' was reported, he was most interested to discover when the butterfly had hatched that it left behind a large red spot in the bottom of the box. This red spot appeared, to de Peiresc, to be a match for the spots that were spinkled across the countryside; and the appearance of the spots coincided with the hatching of the butterflies in the same area, confirming this idea for him.
Following up this idea, de Peiresc examined where the spots were found. They were not in the middle of the cities, nor on the rooftops; nor were they found on surfaces that faced the sky. The greatest numbers of the spots were in cavities that were perfect for the butterflies to occupy, and at a height on the walls that was not more so than the butterflies would normally fly. Having thus been satisfied as to answer, de Peiresc then extended his theory to propose it to be the solution for other historic instances of 'blood rains.'
A Note on the Source
The earliest source for this account I've been able to track was published in 1734, 126 years after the event in question... therefore I can't guarentee this happened. But if it's a false story, it's one that's been around for a very long time!